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1.2.3: Microbes and the Origin of Life on Earth - Biology


Life on Earth is thought to have originated from the oldest single-cell archaea and bacteria.

Learning Objectives

  • Assess the characteristics of pre-life earth and which adaptations allowed early microbial life to flourish.

Key Points

  • The proposed mechanisms for the origin of life on Earth include endosymbiosis and panspermia. Both are debatable theories.
  • In these two theories, bacteria and extremophile archaea are thought to have initiated an oxygenated atmosphere creating new forms of life.
  • Evolutionary processes over billions of years gave rise to the biodiversity of life on Earth.

Key Terms

  • endosymbiosis: A condition of living within the body or cells of another organism.
  • panspermia: The hypothesis that microorganisms may transmit life from outer space to habitable bodies; or the process of such transmission.

Scientific evidence suggests that life began on Earth some 3.5 billion years ago. Since then, life has evolved into a wide variety of forms, which biologists have classified into a hierarchy of taxa. Some of the oldest cells on Earth are single-cell organisms called archaea and bacteria. Fossil records indicate that mounds of bacteria once covered young Earth. Some began making their own food using carbon dioxide in the atmosphere and energy they harvested from the sun. This process (called photosynthesis) produced enough oxygen to change Earth’s atmosphere.

Soon afterward, new oxygen-breathing life forms came onto the scene. With a population of increasingly diverse bacterial life, the stage was set for more life to form. There is compelling evidence that mitochondria and chloroplasts were once primitive bacterial cells. This evidence is described in the endosymbiotic theory. Symbiosis occurs when two different species benefit from living and working together. When one organism actually lives inside the other it’s called endosymbiosis. The endosymbiotic theory describes how a large host cell and ingested bacteria could easily become dependent on one another for survival, resulting in a permanent relationship.

Over millions of years of evolution, mitochondria and chloroplasts have become more specialized and today they cannot live outside the cell. Mitochondria and chloroplasts have striking similarities to bacteria cells. They have their own DNA, which is separate from the DNA found in the nucleus of the cell. And both organelles use their DNA to produce many proteins and enzymes required for their function. A double membrane surrounding both mitochondria and chloroplasts is further evidence that each was ingested by a primitive host. The two organelles also reproduce like bacteria, replicating their own DNA and directing their own division.

Mitochondrial DNA (mtDNA) has a unique pattern of inheritance. It is passed down directly from mother to child, and it accumulates changes much more slowly than other types of DNA. Because of its unique characteristics, mtDNA has provided important clues about evolutionary history. For example, differences in mtDNA are examined to estimate how closely related one species is to another.

Conditions on Earth 4 billion years ago were very different than they are today. The atmosphere lacked oxygen, and an ozone layer did not yet protect Earth from harmful radiation. Heavy rains, lightning, and volcanic activity were common. Yet the earliest cells originated in this extreme environment. Extremophiles archaea still thrive in extreme habitats. Astrobiologists are now using archaea to study the origins of life on Earth and other planets. Because archaea inhabit places previously considered incompatible with life, they may provide clues that will improve our ability to detect extraterrestrial life. Interestingly, current research suggests archaea may be capable of space travel by meteorite. Such an event termed panspermia could have seeded life on Earth or elsewhere.

The presence of archaea and bacteria changed Earth dramatically. They helped establish a stable atmosphere and produced oxygen in such quantities that eventually life forms could evolve that needed oxygen. The new atmospheric conditions calmed the weather so that the extremes were less severe. Life had created the conditions for new life to be formed. This process is one of the great wonders of nature.


Evolution of Life on Earth: 5 Evidences | Biology

The following points highlight the five important evidences that prove the evolution of life on earth. The evidences are: 1. Miller-Urey Experiment 2. Palaeontological Evidences 3. Comparative Anatomy and Morphological Evidences 4. Biochemical Evidences 5. Bio-Geographical Evidences.

1. Miller-Urey Experiment:

Stanley Miller and Harold Urey performed the experiment in 1953.

They demonstrated clearly that ultraviolet radiations or electrical discharges or a combination of these can produce complex organic compounds from a mixture of CH4, NH3, H2O and H2 and the ratio of methane, ammonia and hydrogen was 2: 1: 2 in the experiment Electric discharge was created, in a closed flask containing CH4, NH3, H2 and water vapour at 800°C.

The conditions were set similar to those in the primitive atmosphere in the laboratory. After a week, they observed the presence of complex molecules like sugar, pigments, nitrogen bases, fats and amino acids in the flask.

They proved that the first non-cellular form of life was created about 3 million years ago (mya) and non-cellular bio-molecules exist in the form of RNA, DNA, protein and polysaccharides, etc.

2. Palaeontological Evidences:

It is the study of fossils. Rocks from sediment and cross-section of the earth’s crust depict the arrangement of these sediments one over the other, during a long history of earth.

Sediments of different aged rocks contain fossils of different life forms that lived and died during the formation of a particular segment.

A study of fossil in different sedimentary layers indicates the geological period in which they existed.

The study showed that the life forms varied overtime and certain life forms are restricted to certain geological time scale. Hence, new forms of life have evolved at different times in the history of earth.

3. Comparative Anatomy and Morphological Evidences:

These show the similarities and differences among the organisms of today and those existed years ago. These evidences come from the comparative study of external and internal structure.

These can be determined by the following types:

It is the relation among the organs of different groups of organisms, that show similarity in the basic structure and embryonic development, but have different functions. Homology in organs indicates common ancestry. It is based on divergent evolution. When due to different needs, some structures developed differently, the condition is called divergent evolution. This results in the formation of homologous organs.

Examples of homology are as follows:

(a) Forelimbs of animals like whales, bats and cheetah.

(b) Vertebrate’s heart and brains.

(c) Thorns and tendrils of Bougainvillea and Cucurbita.

It is a situation, exactly opposite to the homology. In analogy, the organs are functionally similar but anatomically different. Convergent evolution is the evolution where different structures develop similarly. This results in analogous organs.

Examples of analogy are as follows:

(i) Wings of butterfly and birds.

(ii) Eyes of Octopus and mammals.

(iii) Flippers of penguins and dolphins.

(iv) Sweet potato (root modification) and potato (stem modification).

4. Biochemical Evidences:

Among diverse organisms, the similarities in proteins and genes perform a common function. The metabolic processes in different organisms are also similar. This occurs due to common ancestry.

5. Bio-Geographical Evidences:

It suggests that the species restricted to a region, develop unique features. Also, species present in widely separated region show similarity of ancestry. Habitat isolation has probably restricted these organisms to a particular geography on the earth.


History of Biology

B iology is the study of life on earth. The History of Biology however, focuses on the advent of life on earth, right from the ancient times. Biological discoveries have a remarkable impact on the human society. Traditionally, the history of biology is diversified into two wings – studies on medicine and theories of natural history. Medicines are not results of current biological discoveries.

Have you heard names like Hippocrates, Aristotle, and Galen of Pergamum? Well, these eminent people were first explorers of the anatomy and physiology of living organisms. Their works focused on the naturalist leanings of organisms, especially animals. Theophrastus, the most notable work of Aristotle still holds a valuable place in the hearts of our modern-day scientists. Do you know why? Theophrastus makes an enormous contribution to the study of zoology, botany, ecology, and taxonomy, all of which are essential branches of biology.

Awareness about medicines became prominent during the middle ages. It is believed that Islamic scholars working by the Galenic and Aristotelian traditions were the first to introduce medicinal science. Neolithic Revolution was a big turning point in the history of biology. This age-old revolution dated 10,000 years ago brought practices of farming and animal husbandry into the limelight.

Much before the study of human beings, biology referred to the study of plant and animal life. Works on botanical studies by Albertus Magnus (1206-1280) and ‘The Art on Falconry,’ introducing the first resource to ornithology by the Holy Roman Emperor Frederick II (1194-1250) played a pivotal role in shaping the natural history of biology.

Botany flourished during the Renaissance and early modern period. Plants were then referred to as ‘materia medica’ because studies proved that plants brimmed with amazing medicinal properties. Not just the Greek culture but ancient cultures of Egypt, China, Mesopotamia, and India had an immense contribution to the evolution of biology. From classical Chinese medicine, formulated by theories by Yin and Yang and the Five Phases to the Indian introduction of Ayurveda, discovery, and study of medicinal sciences became highly popular. Zhuangzi, the noted Taoist philosopher, first brought his ideas about evolution on the boards during the 4th Century. His philosophy stated that species differ in attributes in response to diverse environmental conditions. Developments began springing in gradually during the 17th and 18th Century.

Theories regarding a quantitative approach to physiology and Santorio’s studies on Metabolism ruled the charts. It was only during the 19th Century when several disciplines of biological science were introduced like embryology, cytology, morphology, bacteriology, paleontology, geography and geology.

The roots of Biology, the term coined after combining the Greek words of ‘Bios’ meaning life and ‘Logy’ meaning science dates back to the secular traditions of ancient philosophies. Learning about the history of biology is an attempt to understand the evolution of science.

Here is the history of several branches of biology.

History of Anatomy

History of Biochemistry

History of Biotechnology

History of Botany

History of Cell Biology

History of Ecology

Complete History of Evolution

History of Genetics

History of Immunology

History of Microbiology

Cite This Page

Here are some excellent resources on various historians and scientists who have contributed to biological studies from the dawn of time:


A Brief History of the Human Genome Project
This chapter summarizes human genetics and its history with simple descriptions of modes of inheritance using the commonly-used terms from the genetic literature. It also describes current efforts to create genetic maps and to sequence the 3 billion bases in the human genome.


Biographies – the Scientists
Alphabetical list of scientists, including biologists, each with a precis of the scientists life and achievements. Links to deeper and more extensive materials.

Adam Sedgwick (1785-1873)
Adam Sedgwick was born on March 22, 1785, the third of seven children of an Anglican vicar, in Dent, Yorkshire, England. His home life was happy like so many geologists, young Adam spent time rambling through the countryside, looking at and collecting rocks and fossils. Despite his family’s modest means, Sedgwick attended nearby Sedbergh School, and then entered Trinity College at Cambridge University, as a “sizar” — a type of scholarship student.

Antony van Leeuwenhoek (1632-1723)
Leeuwenhoek was an unlikely scientist. He came from a family of tradesmen, received no higher education, and knew no languages other than his native Dutch.

This would have been enough to exclude him from the scientific community of his time. With skill, diligence, and an open mind free of the scientific dogma, he succeeded in making some of the most important discoveries in biology. It was he who discovered bacteria, free-living and parasitic microscopic protists, and much more.

Aristotle (384-322 B.C.E.)
Though Aristotle’s work in zoology was not without errors, it was the grandest biological synthesis of the time, and remained the ultimate authority for many centuries after his death. His observations on the anatomy of octopus, cuttlefish, crustaceans, and many other marine invertebrates are remarkably accurate, and could only have been made from first-hand experience with dissection.

Biography of Francis Harry Compton Crick
Biography of Francis Harry Compton Crick from the Nobel Foundation.

Biography of James Dewey Watson
Biography of James Dewey Watson from the Nobel Foundation.

Carl Linnaeus (1707-1778)
Carl Linnaeus, also known as Carl von Linné or Carolus Linnaeus, is often called the Father of Taxonomy. His system for naming, ranking, and classifying organisms is still in wide use today (with many changes). His ideas on classification have influenced generations of biologists during and after his own lifetime, even those opposed to the philosophical and theological roots of his work.

DNA from the Beginning
DNA from the Beginning is an animated primer on molecular biology and genetics. It goes through the major discoveries and experiments from Mendel’s peas to the 21st century’s genetic age of the Human Genome Project.

Early Classics in Biogeography, Distribution, and Diversity Studies: To 1950
The following bibliography and full-text archive is designed as a service to advanced students and researchers engaged in work in biogeography, biodiversity, history of science, and related studies. The subjects involved touch on fields ranging from ecology, conservation, systematics and physical geography, to evolutionary biology, cultural biogeography, paleobiology, and bioclimatology–but have in common a relevance to the study of geographical distribution and diversity.

Edward Drinker Cope (1840-1897)
Edward Drinker Cope was an American paleontologist and evolutionist. He was one of the founders of the Neo- Lamarckian school of evolutionary thought. This school believed that changes in developmental (embryonic) timing, not natural selection, was the driving force of evolution. In 1867, Cope suggested that most changes in species occured by coordinated additions to the ontogeny of all the individuals in a species.

Erasmus Darwin (1731-1802)
As a naturalist, Darwin formulated one of the first formal theories on evolution in Zoonomia, or, The Laws of Organic Life (1794-1796). He also presented his evolutionary ideas in verse, in particular in the posthumously published poem The Temple of Nature. Although he did not come up with natural selection, he did discuss ideas that his grandson elaborated on sixty years later, such as how life evolved from a single common ancestor, forming “one living filament”.

Ernst Haeckel (1834-1919)
Ernst Haeckel, much like Herbert Spencer, was always quotable, even when wrong. Although best known for the famous statement “ontogeny recapitulates phylogeny”, he also coined many words commonly used by biologists today, such as phylum, phylogeny, and ecology. On the other hand, Haeckel also stated that “politics is applied biology”, a quote used by Nazi propagandists.

Georges Cuvier (1769-1832)
Without a doubt, Georges Cuvier possessed one of the finest minds in history. Almost single-handedly, he founded vertebrate paleontology as a scientific discipline and created the comparative method of organismal biology, an incredibly powerful tool. It was Cuvier who firmly established the fact of the extinction of past lifeforms. He contributed an immense amount of research in vertebrate and invertebrate zoology and paleontology, and also wrote and lectured on the history of science.

Georges-Louis Leclerc, Comte de Buffon (1707-1788)
100 years before Darwin, Buffon, in his Historie Naturelle, a 44 volume encyclopedia describing everything known about the natural world, wrestled with the similarities of humans and apes and even talked about common ancestry of Man and apes. Although Buffon believed in organic change, he did not provide a coherent mechanism for such changes. He thought that the environment acted directly on organisms through what he called “organic particles“.

Georgius Agricola (1494-1555)
Georg Bauer, better known by the Latin version of his name Georgius Agricola, is considered the founder of geology as a discipline. His work paved the way for further systematic study of the Earth and of its rocks, minerals, and fossils. He made fundamental contributions to mining geology and metallurgy, mineralogy, structural geology, and paleontology.

History of Genetics: Professor Michael Dietrich, Dartmouth College, maintains a web site of useful resources on the history of genetics. Link.

Jean-Baptiste Lamarck (1744-1829)
Lamarck’s scientific theories were largely ignored or attacked during his lifetime Lamarck never won the acceptance and esteem of his colleagues Buffon and Cuvier, and he died in poverty and obscurity. Today, the name of Lamarck is associated merely with a discredited theory of heredity, the “inheritance of acquired traits.” However, Charles Darwin, Lyell, Haeckel, and other early evolutionists acknowledged him as a great zoologist and as a forerunner of evolution.

John Ray (1628-1705)
One of the most eminent naturalists of his time, John Ray was also an influential philosopher and theologian. Ray is often referred to as the father of natural history in Britain.

Leonardo da Vinci (1452-1519)
It may seem unusual to include Leonardo da Vinci in a list of paleontologists and evolutionary biologists. Leonardo was and is best known as an artist. Yet he was far more than a great artist: he had one of the best scientific minds of his time. He carried out research in fields ranging from architecture and civil engineering to astronomy to anatomy and zoology to geography, geology and paleontology.

Louis Agassiz (1807-1873)
One of the great scientists of his day, and one of the “founding fathers” of the modern American scientific tradition, Louis Agassiz remains something of a historical enigma. A great systematist and paleontologist, a renowned teacher and tireless promoter of science in America, he was also a lifelong opponent of Darwin’s theory of evolution. Yet even his most critical attacks on evolution have provided evolutionary biologists with insights.

Louis Pasteur
Louis Pasteur was born on December 27, 1822 in Dole, in the region of Jura, France. His discovery that most infectious diseases are caused by germs, known as the “germ theory of disease”, is one of the most important in medical history. His work became the foundation for the science of microbiology, and a cornerstone of modern medicine.

Mary Anning (1799-1847)
Even though Mary Anning’s life has been made the subject of several books and articles, comparatively little is known about her life, and many people are unaware of her contributions to paleontology in its early days as a scientific discipline. How can someone describe as ‘the greatest fossilist the world ever knew’ be so obscure that even many paleontologists are not aware of her contribution? She was a woman in a man’s England.

Nicholas Steno (1638-1686)
Despite a relatively brief scientific career, Nicholas Steno’s work on the formation of rock layers and the fossils they contain was crucial to the development of modern geology. The principles he stated continue to be used today by geologists and paleontologists.

Patrick Matthew (1790-1874)
He was not a trained scientist, and his evolutionary insights lie buried in the middle of his books and articles on agriculture and politics. Yet he developed a theory of natural selection nearly thirty years before the publication of Darwin’s Origin of Species, with both deep differences and remarkable similarities to Darwin’s theory.

Richard Owen (1804-1892)
Owen synthesized French anatomical work, especially from Cuvier and Geoffroy, with German transcendental anatomy. He gave us many of the terms still used today in anatomy and evolutionary biology, including “homology”. Owen famously defined homology in 1843 as “the same organ in different animals under every variety of form and function.”

Robert Hooke (1635-1703)
His name is somewhat obscure today, due in part to the enmity of his famous, influential, and extremely vindictive colleague, Sir Isaac Newton. Yet Hooke was perhaps the single greatest experimental scientist of the seventeenth century. His interests knew no bounds, ranging from physics and astronomy, to chemistry, biology, and geology, to architecture and naval technology.

The Alfred Russel Wallace Page
My site on Alfred Russel Wallace contains the full-text of over 100 of his writings, extensive bibliographies, and various kinds of commentary. It is one of the largest history of science-oriented sites on the Web.

The History of Cell Biology
Too many colleagues forget what is already known in scientific literature. Acting as independent researchers they have ignored the findings of their predecessors. I discovered by searching the Internet that web-sites often include contradictory descriptions of the same facts or events. If study of scientific history was adequately funded, we would be compelled to write it anew.

The Works of Charles Darwin Online
Links to full-length, online versions of Charles Darwin’s most important books: The Voyage of the Beagle, The Origin of Species, and The Descent of Man.

The World of Richard Dawkins
Richard Dawkins was educated at Oxford University and has taught zoology at the universities of California and Oxford. He is the Charles Simonyi Professor of the Public Understanding of Science at Oxford University. His books about evolution and science include The Selfish Gene, The Extended Phenotype, The Blind Watchmaker, River Out of Eden, Climbing Mount Improbable, and most recently, Unweaving the Rainbow.

Thomas Henry Huxley (1825-1895)
Thomas Henry Huxley was one of the first adherents to Darwin’s theory of evolution by natural selection, and did more than anyone else to advance its acceptance among scientists and the public alike.

Thomas Malthus (1766-1834)
Malthus was a political economist who was concerned about, what he saw as, the decline of living conditions in nineteenth-century England. He blamed this decline on three elements: The overproduction of young the inability of resources to keep up with the rising human population and the irresponsibility of the lower classes. To combat this, Malthus suggested the family size of the lower class ought to be regulated such that low-income families do not produce more children than they can support.

William Paley (1743-1805)
His most influential contribution to biological thought was his book Natural Theology: or, Evidences of the Existence and Attributes of the Deity, Collected from the Appearances of Nature, first published in 1802. In this book, Paley laid out a full exposition of natural theology, the belief that the nature of God could be understood by reference to His creation, the natural world.

Étienne Geoffroy St. Hilaire (1772-1844)
Étienne Geoffroy St. Hilaire was born on April 15, 1772, in Étampes, near Paris, France. Receiving a law degree in 1790, he went on to study medicine and science in Paris, at the College du Cardinal Lemoine. When the Reign of Terror struck, Geoffroy risked his life to save some of his teachers and colleagues from the guillotine. Managing to keep his own head, Geoffroy was appointed a professor of vertebrate zoology at the Jardin des Plantes.


Origin of Life on Earth

The origin of life is a mystery, the ultimate chicken-and-egg conundrum (R Service, 2015). When you and fellow students together discussed the defining characteristics of life, you probably included reproduction and hereditary information, transformation of energy, growth and response to the environment. You may also have said that, at least on Earth, all life is composed of cells, with membranes that form boundaries between the cell and its environment, and that cells were composed of organic molecules (composed of carbon, hydrogen, nitrogen, oxygen, phosphate, and sulfur – CHNOPS). The conundrum is that, on Earth today, all life comes from pre-existing life. Pasteur’s experiments disproved spontaneous generation of microbial life from boiled nutrient broth. No scientist has yet been able to create a living cell from organic molecules. So how could life have arisen on Earth, around 3.8 billion years ago? (Keep in mind the scale of time we’re talking about here – the Earth is 4.6 billion years old, so it took almost a billion years for chemical evolution to result in biological life.) How can this question be addressed using the process of scientific inquiry?

Origin of life studies

Although scientists cannot directly address how life on Earth arose, they can formulate and test hypotheses about natural processes that could account for various intermediate steps, consistent with the geological evidence. In the 1920s, Alexander Oparin and J. B. S. Haldane independently proposed nearly identical hypotheses for how life originated on Earth. Their hypothesis is now called the Oparin-Haldane hypothesis, and the key steps are:

  1. formation of organic molecules, the building blocks of cells (e.g., amino acids, nucleotides, simple sugars)
  2. formation of polymers (longer chains) of organic molecules, that can function as enzymes to carry out metabolic reactions, encode hereditary information, and possibly replicate (e.g., proteins, RNA strands),
  3. formation of protocells concentrations of organic molecules and polymers that carry out metabolic reactions within an enclosed system, separated from the environment by a semi-permeable membrane, such as a lipid bilayer membrane

The Oparin-Haldane hypothesis has been continually tested and revised, and any hypothesis about how life began must account for the 3 primary universal requirements for life: the ability to reproduce and replicate hereditary information the enclosure in membranes to form cells the use of energy to accomplish growth and reproduction.

1. How did organic molecules form on a pre-biotic Earth?

Miller-Urey experiment
Stanley Miller and Harold Urey tested the first step of the Oparin-Haldane hypothesis by investigating the formation of organic molecules from inorganic compounds. Their 1950s experiment produced a number of organic molecules, including amino acids, that are made and used by living cells to grow and replicate.

Miller-Urey experiment, Wikimedia Commons illustration by Adrian Hunter

Miller and Urey used an experimental setup to recreate what environmental conditions were believed to be like on early Earth. A gaseous chamber simulated an atmosphere with reducing compounds (electron donors) such as methane, ammonia and hydrogen. Electrical sparks simulated lightning to provide energy. In only about a week’s time, this simple apparatus caused chemical reactions that produced a variety of organic molecules, some of which are the basic building blocks of life, such as amino acids. Although scientists no longer believe that pre-biotic Earth had such a reducing atmosphere, such reducing environments may be found in deep-sea hydrothermal vents, which also have a source of energy in the form of the heat from the vents. In addition, more recent experiments – that used conditions that are thought to better reflect the conditions of early Earth – have also produced a variety of organic molecules including amino acids and nucleotides (the building blocks of RNA and DNA) (McCollom, 2013).

The video below gives a nice overview of the rationale, setup, and findings from the Miller-Urey experiment (although it incorrectly overstates that Darwin showed that relatively simple creatures can gradually give rise to more complex creatures).

Organic molecules from meteors

Each day the Earth is bombarded with meteorites and dust from comets. Analyses of space dust and meteors that have landed on Earth have revealed that they contain many organic molecules. The in-fall of cometary dust and meteorites was far greater when the Earth was young (4 billion years ago). Many scientists believe that such extra-terrestrial organic matter contributed significantly to the organic molecules available at the time that life on Earth began. The figure below from Bernstein 2006 shows the 3 major sources of organic molecules on pre-life Earth: atmospheric synthesis by Miller-Urey chemistry, synthesis at deep-sea hydrothermal vents, and in-fall of organic molecules synthesized in outer space.

2. Formation of organic polymers

Given a high enough concentration of these basic organic molecules, under certain conditions these will link together to form polymers (chains of molecules covalently bonded together). For example, amino acids link together to form polypeptide chains, that fold to become protein molecules. Ribose, a 5-carbon sugar, can bond with a nitrogenous base and phosphate to a nucleotide. Nucleotides link together to form nucleic acids, like DNA and RNA. While this is accomplished now by enzymes in living cells, polymerization of organic molecules can also be catalyzed by certain types of clay or other types of mineral surfaces. Experiments testing this model have produced RNA molecules up to 50-units long, in only a 1-2 week period of time (Ferris, 2006).

Enzymatic activity and hereditary information in one polymer: the RNA World hypothesis

The discovery by Thomas Cech that some RNA molecules can catalyze their own site-specific cleavage led to a Nobel prize (for Cech and Altman), the term “ribozymes” to denote catalytic RNA molecules, and the revival of a hypothesis that RNA molecules were the original hereditary molecules, pre-dating DNA. For origin-of-life researchers, here was the possibility that RNA molecules could both encode hereditary information, and catalyze their own replication. DNA as the first hereditary molecule posed real problems for origin-of-life researchers because DNA replication requires protein enzymes (DNA polymerases) and RNA primers (see page on DNA replication), so it’s difficult to envision how such a complex hereditary system could have evolved from scratch. With catalytic RNA molecules, a single molecule or family of similar molecules could potentially store genetic information and replicate themselves, with no proteins needed initially.

Populations of such catalytic RNA molecules would undergo a molecular evolution conceptually identical to biological evolution by natural selection. RNA molecules would make copies of each other, making mistakes and generating variants. The variants that were most successful at replicating themselves (recognize identical or very similar RNA molecules and most efficiently replicate them) would increase in frequency in the population of catalytic RNA molecules. The RNA world hypothesis envisions a stage in the origin of life where self-replicating RNA molecules eventually led to the evolution of a hereditary system in the first cells or proto-cells. A system of RNA molecules that encode codons to specify amino acids, and tRNA-like molecules conveying matching amino acids, and catalytic RNAs that create peptide bonds, would constitute a hereditary system much like today’s cells, without DNA.

At some point in the lineage leading to the Last Universal Common Ancestor, DNA became the preferred long-term storage molecule for genetic information. DNA molecules are more chemically stable than RNA (deoxyribose is more chemically inert than ribose). Having two complementary strands means that each strand of DNA can serve as a template for replication of its partner strand, providing some innate redundancy. These and possibly other traits gave cells with a DNA hereditary system a selective advantage so that all cellular life on Earth uses DNA to store and transmit genetic information.

Still, even today, ribozymes play universal and central roles in cellular information processing. The ribosome is a large complex of RNAs and proteins that reads the genetic information in a strand of RNA to synthesize proteins. The key catalytic activity, the formation of peptide bonds to link two amino acids together, is catalyzed by a ribosomal RNA molecule. The ribosome is a giant ribozyme. Since ribosomes are universal to all cells, such catalytic RNAs must have been present in the Last Universal Common Ancestor of all current life on Earth.

Visit the http://exploringorigins.org/ribozymes.html page to view the first ribozyme from Tetrahymena, discovered by Tom Cech, and the structure of the ribosomal RNAs.

The http://exploringorigins.org/nucleicacids.html page has videos of polymerization of RNA from nucleotides, template-directed RNA synthesis, and a model of RNA self-replication.

The video below explains the rationale behind the RNA world hypothesis and briefly describes some of the findings from different RNA world experiments.

3. Protocells: self-replicating and metabolic enzymes in a bag

All life on Earth is composed of cells. Cells have lipid membranes that separate their inner contents, the cytoplasm, from the environment. The lipid membranes allow cells to maintain high concentrations of molecules like nucleotides needed for self-replicating RNAs to function more efficiently. Cells also maintain large differences in concentration (concentration gradients) of ions across the membrane to drive transport processes and cellular energy metabolism.

Lipids are hydrophobic, and will spontaneously self-assemble in water to form either micelles or lipid bilayer vesicles. Vesicles that enclose self-replicating RNAs and other enzymes, take in reactants across the membrane, export products, grow by accretion of lipid micelles, and divide by fission of the vesicle, are called proto-cells or protobionts and may have been the precursors of cellular life.

The video below explores the differences between chemical and biological evolution, and highlights proto-cells as an example of chemical evolution.

At what point would evolutionary processes, such as natural selection, begin to drive the origin of the first cells?

Biological evolution is restricted to living organisms. So once the first cells, complete with a hereditary system, were formed, they would be subject to evolutionary processes, and natural selection would drive adaptation to their local environments, and populations in different environments would undergo speciation as gene flow becomes restricted between isolated populations.

However, the RNA World Hypothesis envisions evolutionary processes driving populations of self-replicating RNA molecules or proto-cells containing such RNA molecules. RNA molecules that replicated imperfectly would produce daughter molecules with slightly different sequences. The ones that replicate better, or improve the growth replication of their host proto-cells, would have more progeny. Hence, molecular evolution of self-replicating RNA molecules or proto-cell populations containing self-replicating RNA molecules would favor the eventual formation of the first cells.

References and Resources

Bernstein M 2006. Prebiotic materials from on and off the early Earth. Philos Trans
R Soc Lond B Biol Sci. 361:1689-700 discussion 1700-2. PubMed
PMID: 17008210 PubMed Central PMCID: PMC1664678.


20.1 Organizing Life on Earth

By the end of this section, you will be able to do the following:

  • Discuss the need for a comprehensive classification system
  • List the different levels of the taxonomic classification system
  • Describe how systematics and taxonomy relate to phylogeny
  • Discuss a phylogenetic tree's components and purpose

In scientific terms, phylogeny is the evolutionary history and relationship of an organism or group of organisms. A phylogeny describes the organisim's relationships, such as from which organisms it may have evolved, or to which species it is most closely related. Phylogenetic relationships provide information on shared ancestry but not necessarily on how organisms are similar or different.

Phylogenetic Trees

Scientists use a tool called a phylogenetic tree to show the evolutionary pathways and connections among organisms. A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or groups of organisms. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since one cannot go back to confirm the proposed relationships. In other words, we can construct a “tree of life” to illustrate when different organisms evolved and to show the relationships among different organisms (Figure 20.2).

Unlike a taxonomic classification diagram, we can read a phylogenetic tree like a map of evolutionary history. Many phylogenetic trees have a single lineage at the base representing a common ancestor. Scientists call such trees rooted , which means there is a single ancestral lineage (typically drawn from the bottom or left) to which all organisms represented in the diagram relate. Notice in the rooted phylogenetic tree that the three domains— Bacteria, Archaea, and Eukarya—diverge from a single point and branch off. The small branch that plants and animals (including humans) occupy in this diagram shows how recent and miniscule these groups are compared with other organisms. Unrooted trees do not show a common ancestor but do show relationships among species.

In a rooted tree, the branching indicates evolutionary relationships (Figure 20.3). The point where a split occurs, a branch point , represents where a single lineage evolved into a distinct new one. We call a lineage that evolved early from the root that remains unbranched a basal taxon . We call two lineages stemming from the same branch point sister taxa . A branch with more than two lineages is a polytomy and serves to illustrate where scientists have not definitively determined all of the relationships. Note that although sister taxa and polytomy do share an ancestor, it does not mean that the groups of organisms split or evolved from each other. Organisms in two taxa may have split at a specific branch point, but neither taxon gave rise to the other.

The diagrams above can serve as a pathway to understanding evolutionary history. We can trace the pathway from the origin of life to any individual species by navigating through the evolutionary branches between the two points. Also, by starting with a single species and tracing back towards the "trunk" of the tree, one can discover species' ancestors, as well as where lineages share a common ancestry. In addition, we can use the tree to study entire groups of organisms.

Another point to mention on phylogenetic tree structure is that rotation at branch points does not change the information. For example, if a branch point rotated and the taxon order changed, this would not alter the information because each taxon's evolution from the branch point was independent of the other.

Many disciplines within the study of biology contribute to understanding how past and present life evolved over time these disciplines together contribute to building, updating, and maintaining the “tree of life.” Systematics is the field that scientists use to organize and classify organisms based on evolutionary relationships. Researchers may use data from fossils, from studying the body part structures, or molecules that an organism uses, and DNA analysis. By combining data from many sources, scientists can construct an organism's phylogeny Since phylogenetic trees are hypotheses, they will continue to change as researchers discover new types of life and learn new information.

Limitations of Phylogenetic Trees

It may be easy to assume that more closely related organisms look more alike, and while this is often the case, it is not always true. If two closely related lineages evolved under significantly varied surroundings, it is possible for the two groups to appear more different than other groups that are not as closely related. For example, the phylogenetic tree in Figure 20.4 shows that lizards and rabbits both have amniotic eggs whereas, frogs do not. Yet lizards and frogs appear more similar than lizards and rabbits.

Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for length of time, only the evolutionary order. In other words, a branch's length does not typically mean more time passed, nor does a short branch mean less time passed— unless specified on the diagram. For example, in Figure 20.4, the tree does not indicate how much time passed between the evolution of amniotic eggs and hair. What the tree does show is the order in which things took place. Again using Figure 20.4, the tree shows that the oldest trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a part of the greater whole, and like a real tree, it does not grow in only one direction after a new branch develops. Thus, for the organisms in Figure 20.4, just because a vertebral column evolved does not mean that invertebrate evolution ceased. It only means that a new branch formed. Also, groups that are not closely related, but evolve under similar conditions, may appear more phenotypically similar to each other than to a close relative.

Link to Learning

Head to this website to see interactive exercises that allow you to explore the evolutionary relationships among species.

Classification Levels

Taxonomy (which literally means “arrangement law”) is the science of classifying organisms to construct internationally shared classification systems with each organism placed into increasingly more inclusive groupings. Think about a grocery store's organization. One large space is divided into departments, such as produce, dairy, and meats. Then each department further divides into aisles, then each aisle into categories and brands, and then finally a single product. We call this organization from larger to smaller, more specific categories a hierarchical system.

The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish botanist, zoologist, and physician) uses a hierarchical model. Moving from the point of origin, the groups become more specific, until one branch ends as a single species. For example, after the common beginning of all life, scientists divide organisms into three large categories called domains: Bacteria, Archaea, and Eukarya. Within each domain is a second category called a kingdom . After kingdoms, the subsequent categories of increasing specificity are: phylum , class , order , family , genus , and species (Figure 20.5).

The kingdom Animalia stems from the Eukarya domain. Figure 20.5 above shows the classification for the common dog. Therefore, the full name of an organism technically has eight terms. For the dog it is: Eukarya, Animalia, Chordata, Mammalia, Carnivora, Canidae, Canis, and lupus. Notice that each name is capitalized except for species, and the genus and species names are italicized. Scientists generally refer to an organism only by its genus and species, which is its two-word scientific name, or binomial nomenclature . Therefore, the scientific name of the dog is Canis lupus. The name at each level is also a taxon . In other words, dogs are in order Carnivora. Carnivora is the name of the taxon at the order level Canidae is the taxon at the family level, and so forth. Organisms also have a common name that people typically use, in this case, dog. Note that the dog is additionally a subspecies: the “familiaris” in Canis lupus familiaris. Subspecies are members of the same species that are capable of mating and reproducing viable offspring, but they are separate subspecies due to geographic or behavioral isolation or other factors.

Figure 20.6 shows how the levels move toward specificity with other organisms. Notice how the dog shares a domain with the widest diversity of organisms, including plants and butterflies. At each sublevel, the organisms become more similar because they are more closely related. Historically, scientists classified organisms using characteristics, but as DNA technology developed, they have determined more precise phylogenies.

Visual Connection

At what levels are cats and dogs part of the same group?

Link to Learning

Visit this website to explore the classifications of thousands of organisms. This reference site contains about 10% of the described species on the planet.

Recent genetic analysis and other advancements have found that some earlier phylogenetic classifications do not align with the evolutionary past therefore, researchers must make changes and updates as new discoveries occur. Recall that phylogenetic trees are hypotheses and are modified as data becomes available. In addition, classification historically has focused on grouping organisms mainly by shared characteristics and does not necessarily illustrate how the various groups relate to each other from an evolutionary perspective. For example, despite the fact that a hippopotamus resembles a pig more than a whale, the hippopotamus may be the whale's closest living relative.

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    Definition of Life from a Biological Perspective

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    Find a media piece that recognizes the fundamental concepts of chemistry in biology and describe how it helps to better understand how fundamental concepts of chemistry affect biology.

    Find a media piece that describes the energy metabolism and how it helps to better understand the energy metabolism of cells.

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    What is the meaning of life after finding this information.

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    Find any media piece (article, video, presentation, song, or other) related to scientific method and describe how it helped to better understand how the scientific method is used to create hypotheses and experiments.
    Here are the articles related to the scientific method:
    http://www.livescience.com/20896-science-scientific-method.html
    http://www.historyofvaccines.org/content/articles/scientific-method-vaccine-history
    History of Science
    In the late 1400s, the painter Leonardo da Vinci gather data on the human body and wants to find evidence that the human body is microcosm. Artist, scientist, and mathematician gathers information about how optics and hydrodynamics ever worked.

    In the late 1500s, Copernicus had discovered the model in which Earth and planets revolve around the sun called heliocentrism. " (1) This model form the center of our solar system. (1)
    Johannes Kepler proposed " the laws of planetary motion, and Galileo invented the telescope that is used to study the sun and planets." (1) Isaac Newton has developed " his laws of motion in the 1600s. "(1)
    In the 1700s, Ben Franklin discovered "electricity in the lightning, and Antoine Lavoisier developed the law of conservation of mass. " (1) John Dalton introduced the "atomic theory that stated that all matter is comprised of atoms that combined to form molecules. "(1)
    Gregor Mendels "advanced the basis of modern genetics in his laws of inheritance ." (1)
    Albert Einstein is "best known for his theory of relativity, which dominate in the beginning of the 20th century." (1)
    In the 21st century, James D. Watson and Francis Crick "had discovered the structure of DNA." (1) At the same time, the first draft of the human genome was completed leading to the extensive understanding of DNA and advancing the study of genetics and their role in human biology as a predictor of diseases and other disorders.
    These scientific discoveries are the basis for how the scientific method led to their discoveries.
    The basics for the scientific method are that:
    1. "State or make an observation or observations." (1)
    2. "Ask questions about the observations and gather data about them." (1)
    3. "Form a hypothesis or a tentative description of what is been observed and make predictions based on that hypothesis." (1)
    4. "Test the predictions or hypothesis in an experiment that can be reproduced."(1)
    5. "Analyze the your data and draw conclusions." (1)
    6. "You can accept or reject the hypothesis or modify the hypothesis if necessary." (1)
    7. " Reproduce the experiment until there are no discrepancies between observations and theory." (1)
    In the scientific method,
    1. "The hypothesis must be testable and falsifiable."(1)
    2. The research must "involve deductive reasoning, not inductive reasoning. " (1) Deductive reasoning is "the process of using true premises to reach a logical true conclusion." (1)
    3. The experiment must "include an independent variable and a dependent variable. " (1) "An independent variable does not change and depend on the dependent variable. "(1)
    4. An experiment "should have an experimental group and a control group. " (1) "The experimental group is compared against the control group. " (1)
    A hypothesis can become a "theory if there are extensive testing that must occur throughout multiple disciplines by separate or different groups of scientists." (1) In science, " a theory is something that is very well supported by observation and experimentation. " (1)
    When " conducting a research project, scientists must observe the scientific method to collect measurable data in an experiment related to a hypothesis. The results aimed to support or contradict a theory." (1, 2)

    Example of using the Scientific Method

    Science requires strict and careful observation that points to an interesting question. One observation that points to an interesting question is by the Scottish biologist Alexander Fleming in the 1920s. Alexander Fleming observe that plates of bacteria that he grew were contaminated with some mould. ( 1, 2, 3) The area around the mould looked free of bacteria growth. ( 1,2,3) He began to form a cause and effect relationship that the mould may prevent the bacterial growth. ( 2,3) His observation led to a conclusion that penicillin could be used to treat bacterial infections. (1, 2,3)

    Hypothesis.
    A hypothesis is a proposal generated by an observation.(1, 2, 3) In Fleming's investigation of antibiotic of mould, the hypothesis form is that if "bacteria were introduced to the filtrates from mould, the bacteria will die." (2)
    "Good hypothesis are testable and falsifiable. " (2, 3) The hypothesis can be subjected to an "observable test and can be proven to be false."(2) For example, in the Fleming's studies of mould, the "hypothesis can be falsified because a test in which bacteria can grew in the presence of the filtrate of the mould would have disproven the hypothesis. " (2, 3)

    Testing
    Scientific studies involve a "test with a control group and an experimental group. " (2, 3) The scientist conducts the experiment on the control group as well as the experimental group. (2, 3) The only difference is that the investigators does not subject the control group to the factor being tested, which is the variable. (2)
    In an experiment testing Fleming's hypothesis, a scientist will introduce "filtrates of mould to cultures of bacteria on glass plates, which serve as an experimental group. " (2, 3) A control group can "contain cultures of bacteria with no addition of mould filtrates." (2, 3) Both groups will be subject to exactly the same conditions. (2)

    Conclusion
    A conclusion involves "analysis and interpretation of the data gathered during the testing phase, which allowed researchers to form conclusion based on the data." (2, 3) A valid conclusion will reflect the hypothesis. (2)

    Find a media piece that recognizes the fundamental concepts of chemistry in biology and describe how it helps to better understand how fundamental concepts of chemistry affect biology.

    Here is a media on the fundamental concepts of chemistry in .

    Solution Summary

    The definition of life from a biological perspectives are given. The media piece that recognizes the fundamental concepts of chemistry in biology are provided.


    After the Miller-Urey experiment: Exploring proteins and membranes

    While ideas about Earth’s primordial atmosphere were in flux from the 1970s onward, NASA’s exploration of the outer Solar System revealed some amazing things about the moons orbiting Jupiter and Saturn. In particular, the space probes Voyager 1, Voyager 2, and Cassini and an atmospheric entry probe to Saturn’s moon Titan called the Huygens probe revealed the exact makeup of Titan’s atmosphere. This inspired other scientists, such as Carl Sagan, to redo Miller’s 1952 experiment with a Titan atmospheric mixture. This too produced important biological compounds. Thus, today, the moon Titan is a prime focus for astrobiology studies in the Solar System. It may have exotic life forms, or it may be a model of how Earth was prior to life.

    Several years after the original Miller-Urey experiment, another investigator, Sidney Fox, ran experiments showing that some of the Miller-Urey compounds – the amino acids – could join together to form polymers, bigger molecules known as peptides, or small proteins. This happened when amino acids made through a Miller-Urey mechanism were splashed onto surfaces of clays and other materials, under hot, dry conditions. On the ancient Earth, such conditions would have occurred at the boundary between ancient ponds or seas and ancient land. Given enough time, complex proteins could arise.

    Other researchers later found that spheres of lipids (the class of organic molecules that includes fats) also could form under conditions thought to exist on the ancient Earth. This would create a water environment inside the sphere that was separated from the outside. In other words, crude membranes can form spontaneously under the same conditions in which biological compounds like amino acids and small proteins can form. The fact that membranes can form spontaneously is key to origins of life research. This is because to move from non-living chemistry to biology, very complex networks of chemical reactions need to emerge. Like a car being made on an assembly line, biological molecules are put together section by section. They also are converted into different molecules section by section, so there is a series of intermediate chemicals in addition to a starting molecule (called a substrate) and final product of each reaction.

    In an open environment like Haldane’s primordial soup, or in an ocean, the various intermediates would simply diffuse away before the chemical pathway had a chance to evolve. But a membrane would enclose all of the chemicals within a compartment. That compartment would then act as a chemical laboratory, holding inside any reactions that happened to emerge. Since we know that membrane spheres can spontaneously form, the primordial soup of early Earth must have had billions of these little chemical laboratories in which the chemistry of life was sputtering along.

    Why are membranes so important in origins of life research?


    How Did Multicellular Life Evolve?

    Scientists are discovering ways in which single cells might have evolved traits that entrenched them into group behavior, paving the way for multicellular life. These discoveries could shed light on how complex extraterrestrial life might evolve on alien worlds.

    Researchers detailed these findings in the October 24, 2016 issue of the journal Science.

    The first known single-celled organisms appeared on Earth about 3.5 billion years ago, roughly a billion years after Earth formed. More complex forms of life took longer to evolve, with the first multicellular animals not appearing until about 600 million years ago.

    The evolution of multicellular life from simpler, unicellular microbes was a pivotal moment in the history of biology on Earth and has drastically reshaped the planet’s ecology. However, one mystery about multicellular organisms is why cells did not return back to single-celled life.

    “Unicellularity is clearly successful — unicellular organisms are much more abundant than multicellular organisms, and have been around for at least an additional 2 billion years,” said lead study author Eric Libby, a mathematical biologist at the Santa Fe Institute in New Mexico. “So what is the advantage to being multicellular and staying that way?”

    The answer to this question is usually cooperation, as cells benefitted more from working together than they would from living alone. However, in scenarios of cooperation, there are constantly tempting opportunities “for cells to shirk their duties — that is, cheat,” Libby said.

    “As an example, consider an ant colony where only the queen is laying eggs and the workers, who cannot reproduce, must sacrifice themselves for the colony,” Libby said. “What prevents the ant worker from leaving the colony and forming a new colony? Well, obviously the ant worker cannot reproduce, so it cannot start its own colony. But if it got a mutation that enabled it to do that, then this would be a real problem for the colony. This kind of struggle is prevalent in the evolution of multicellularity because the first multicellular organisms were only a mutation away from being strictly unicellular.”

    Experiments have shown that a group of microbes that secretes useful molecules that all members of the group can benefit from can grow faster than groups that do not. But within that group, freeloaders that do not expend resources or energy to secrete these molecules grow fastest of all. Another example of cells that grow in a way that harms other members of their groups are cancer cells, which are a potential problem for all multicellular organisms.

    Indeed, many primitive multicellular organisms probably experienced both unicellular and multicellular states, providing opportunities to forego a group lifestyle. For example, the bacterium Pseudomonas fluorescens rapidly evolves to generate multicellular mats on surfaces to gain better access to oxygen. However, once a mat has formed, unicellular cheats have an incentive to not produce the glue responsible for mat formation, ultimately leading to the mat’s destruction.

    To solve the mystery of how multicellular life persisted, scientists are suggesting what they call “ratcheting mechanisms.” Ratchets are devices that permit motion in just one direction. By analogy, ratcheting mechanisms are traits that provide benefits in a group context but are detrimental to loners, ultimately preventing a reversion to a single-celled state, said Libby and study co-author William Ratcliff at the Georgia Institute of Technology in Atlanta.

    In general, the more a trait makes cells in a group mutually reliant, the more it serves as a ratchet. For instance, groups of cells may divide labor so that some cells grow one vital molecule while other cells grow a different essential compound, so these cells do better together than apart, an idea supported by recent experiments with bacteria.

    Ratcheting can also explain the symbiosis between ancient microbes that led to symbionts living inside cells, such as the mitochondria and chloroplasts that respectively help their hosts make use of oxygen and sunlight. The single-celled organisms known as Paramecia do poorly when experimentally derived of photosynthetic symbionts, and in turn symbionts typically lose genes that are required for life outside their hosts.

    These ratcheting mechanisms can lead to seemingly nonsensical results. For instance, apoptosis, or programmed cell death, is a process by which a cell essentially undergoes suicide. However, experiments show that higher rates of apoptosis can actually have benefits. In large clusters of yeast cells, apoptotic cells act like weak links whose death allows small clumps of yeast cells to break free and go on to spread elsewhere where they might have more room and nutrients to grow.

    “This advantage does not work for single cells, which meant that any cell that abandoned the group would suffer a disadvantage,” Libby said. “This work shows that a cell living in a group can experience a fundamentally different environment than a cell living on its own. The environment can be so different that traits disastrous for a solitary organism, like increased rates of death, can become advantageous for cells in a group.”

    When it comes to what these findings mean in the search for alien life, Libby said this research suggests that extraterrestrial behavior might appear odd until one better understands that an organism may be a member of a group.

    “Organisms in communities can adopt behaviors that would appear bizarre or counterintuitive without proper consideration of their communal context,” Libby said. “It is essentially a reminder that a puzzle piece is a puzzle until you know how it fits into a larger context.”

    Libby and his colleagues plan to identify other ratcheting mechanisms.

    “We also have some experiments in the works to calculate the stability provided by some possible ratcheting traits,” Libby said.

    Sign-up to get the latest in news, events, and opportunities from the NASA Astrobiology Program.


    Could Life Have Begun Elsewhere?

    Could life have begun elsewhere? The simplest place to look is in the solar system and compare other planets with Earth. Scientists now have a better understanding of where life exists on Earth, and it is much more widely distributed than we might have guessed. Bacterial life exists over a remarkable temperature range, from near 0ଌ (32ଏ) on melting snow to over 115ଌ (239ଏ) in submarine hydrothermal vents. It exists in acidic environments as strong as battery acid or as alkaline as household ammonia. Bacterial life exists in the dark, in the absence of oxygen, and has even been found growing in the radioactive water of nuclear reactors. In fact, the only constant is that microbial life requires liquid water, and if liquid water exists elsewhere we might expect that life could have started as it did on Earth, and may even still be flourishing.

    Where in the solar system might one find liquid water? There are only two places that scientists know of: Mars and Europa. Mars certainly has water, but in the form of ice. Liquid water cannot exist for long on the surface of Mars, due to the cold temperature and low atmospheric pressure, but it could be locked up in ice beneath the surface, just as water is present in the permafrost of Arctic tundras. Recent images from the Mars Global Surveyor clearly show that liquid water occasionally breaks through the ice and pours down steep slopes on the edges of craters. Europa, a moon of Jupiter about the size of Earth's moon, also has water in the form of a thick sheet of ice, and beneath the ice is a global ocean of liquid water. On both Mars and Europa there is a distinct possibility that life similar to bacteria could be present, and future space missions may finally answer the age-old question: Does life exist elsewhere?