Life on Earth

Some scientists now believe that comets, meteorites, and interplanetary dust deliver organic compounds created in interstellar space to newly formed planets, and that such space-born substances could have "kick-started" the development of life on Earth. Researchers working with NASA's Astrobiology Institute have created "proto-cells" that mimic the membranous structures used to create the living cells found on our planet. They subjected icy dust particles that are rich in organic compounds (i.e., water, methanol or wood alcohol, ammonia, and carbon monoxide) that are found in dense molecular clouds of interstellar space to the harsh conditions found there, such as intense cold and ultraviolet light. An abundance of such structures raining down on wet areas of Earth during its early years could have been important in protecting self-replicating molecules (i.e., the precursors of RNA and DNA) that became encapsulated within them, and eventually such proto-cells could have evolved into primitive lifeforms. (See a NASA summary.) In 2006, two scientists argued that the development of life on Earth was the necessary consequence of available energy built up by geological processes (including polyphosphates made in volcanic processes) on the early Earth, in the same way that lightning relieves the accumulation of electrical charge in thunderclouds (Morowitz and Smith, 2006; and Phillip Ball, Nature, November 14, 2006).

NASA Astrobiology Institute -- larger and detailed images

Ionizing radiation such as ultraviolet light "processes" substances
found in the cold molecular clouds of interstellar space such as
ices rich in organic compounds into even more complex organic
materials. When these processed "residues" are placed in water,
one or more of the compounds present spontaneously form
membranes that produce "proto-cells" ("vesicles").

Years 0.1 to 0.8 Billion

Initially, the Earth's surface was mostly molten rock that gradually cooled through the radiation of heat into space. The primeval atmosphere was composed mostly of water (H₂O), carbon dioxide (CO₂) and monoxide (CO), molecular nitrogen (N₂) and molecular hydrogen (H₂), and hydrogen chloride (HCl) outgassed from molten rock, with only traces of reactive molecular oxygen (O₂). This steamy atmosphere was rich with water released from hydrated minerals and cometary impactors. As the Earth continued to cool from Years 0.1 to 0.3 billion, a torrential rain fell that turned to steam upon hitting the still hot surface, then superheated water, and finally collected into hot or warm seas and oceans above and around cooling crustal rock leaving sediments. Every once in a while, however, a large asteroid or comet would strike the planet which remelted crustal rock and turned oceans back into hot mist. Eventually, a stable rocky crust may have developed between Years 0.2 and 0.4 billion (see J. Bret Bennington's discussion of recycled zircons (crystals of zirconium silicate) from the rocks of western Australia in the Hadean Eon and the January 11, 2001 announcement of zircons found north of Perth that appear to be 4.4 billion years old), covered and surrounded by soupy water that was already rich with organic compounds from interstellar space.

John W. Valley, NSF



Larger, annotated cathodoluminescence image.



The oldest fragment of Earth's primeval crust is a zircon
dated to be 4.4 billion years old, having formed less
than 160 million years after planetary formation (more).

On July 3, 2008, a team of scientists published results (Nemchin et al, 2008) on finding unusually high "light-carbon" isotope ratios possibly indicative of biological origin found in micrometer-sized diamond and graphite inclusions that were later incorporated within 22 zircons from the Jack Hills of Western Australia, which were formed under the pressure of 100 to 150 kilometers (62 to 93 miles) of crustal rock. As the zircons were radioactively dated to be as old as 4.25 billion years, the new findings suggest that carbon-based life may have been present on Earth within the first 300 million years after planetary formation. The ratios of carbon-12 to carbon-13 found within the zircons were unusually high, where such high abundances of carbon-12 are commonly attributed to the presence of organic material created by Earth life. Although some non-biological chemical reactions can create such high light-carbon ratios, those found were so skewed towards carbon-12 that it's unclear exactly how such reactions could have created such abundances. Alternatively, some hypothesize that the reservoir of light carbon on the early Earth indicates the presence of simple organic compounds -- possibly brought by asteroidal or cometary impactors -- that created a hospitable environment for the later emergence of life (more discussion in Rachel Courtland, New Scientist, July 2, 2008; Sid Perkins, Science News, July 2, 2008; and Jonathan Fildes, BBC News, July 2, 2008).

NASA




Larger image.




Earth was under intense bombardment from
cometary and asteroidal impactors, like
stoney asteroid 951 Gaspra, during the first
700 million years after formation.

Despite comparatively intense bombardment by large impactors, chemical and radio-isotopic trace evidence of what appears to be biologically processed carbon in Earth's oldest surviving rocks -- from western Greenland's Isua greenstone belt that are as old as 3.85 billion years -- suggest that self-replicating, carbon-based microbial life became well developed during Earth's first billion years of existence. Although the evidence was subsequently contested, some single-celled microbial life lacking a nucleus that segregates their internal DNA or RNA ("prokaryotes") from the surrounding cytoplasm may have flourished in darkness within cracks in Earth's seafloor crust and around deep, boiling hot ocean springs (known as volcanic vents or "black smokers") without a need for light or free oxygen in the oceans or atmosphere. Adapted to their very hot but watery environment, these microbes metabolized hydrogen-rich compounds or dead or live organic materials to derive the energy that sustains anaerobic life, including sulfate-reducing bacteria that produce Hydrogen Sulfide (H₂S), fermentative bacteria that produce carbon dioxide and alcohol (-OH), and methanogenic bacteria -- the methanogens found in sewage and mudflats today -- that produce methane (CH₄) gas as a waste product.

Dudley Foster, U.S. Geological Survey -- NASA smoker image
("Black smoker" discovered in East Pacific by submersible Alvin)

Although the early Earth was mostly devoid of molecular oxygen, high volcanic activity released significant amounts of molecular hydrogen. With little oxygen available to convert that hydrogen into water, hydrogen gas probably accumulated in the atmosphere and oceans in concentrations as high as hundreds to thousands of parts per million. Thus, the early Earth was likely a paradise for methanogens that feed directly on hydrogen and carbon dioxide, at least until the atmospheric hydrogen was depleted. On the other hand, many anaerobic microbes including methanogens are easily poisoned by oxygen, and the recent discovery of banded sediments with rusted iron on Akilia Island in West Greenland suggests that oxygen-producing microbes living on the surface of wet areas to gather sunlight may have developed by the end of this geologic period (3.85 billion years ago) despite continuing bombardment from space.

Years 0.8 to 2.1 Billion

Diminishment of cometary and meteoric bombardment allowed anaerobic microbes to spread widely in wet habitats. Life diversified and adapted to new biotic niches -- some on land -- but stayed single-celled. Some scientists now believe that anaerobic methanogens began to prosper and eventually filled Earth's atmosphere with nearly 600 times as much methane as they do today. That extra methane would have produced a greenhouse effect strong enough to heat the planet to a higher average temperature than it is today, although the Sun was around 20 percent dimmer at that time (Pavlov et al, 2000). The higher temperatures also led to more humid conditions made possible by a more active water cycle, resulting in a climate that is preferred by many methanogens. The more active water cycle, however, also enhanced the weathering of rocks on early continents which should have pulled enough carbon dioxide out of the atmosphere that its concentration would have fallen until the gas existed in nearly equal amounts with methane. Fortunately, some of the methane tends to form complex hydrocarbons that condensed into dustlike particles to produce a high-altitude haze which absorbed and reradiated incoming sunlight back into space to act as a break on greenhouse heating (James F. Kasting, Scientific American, July 2004).

Cyanosite -- NASA image of Chroococcidiopsis


Dividing Chroococcus sp., a type of cyanobacteria,
photosynthetic microbes that also produce oxygen.
While "primitive," Chroococcidiopsis survives in
extremely dry, cold, and salty environments.

By the end of this period, however, microbes with the ability to produce oxygen were becoming widespread, releasing large quantities of this reactive molecular gas into the oceans and atmosphere (more). Many of these microbes persist today; for example, blue-green (cyanobacteria) or bright green, photosynthetic bacteria use light from the Sun to and chlorophyll convert carbon dioxide and water into "free" molecular oxygen and carbon, made into essential organic substances such as carbohydrates. Other bacteria use bacteriochlorophyll and other photosynthetic proteins to convert light to metabolic energy.

NASA Astrobiology Institute and Penn State
Astrobiology Research Center (PSARC)


Hot spring microbial mats at Yellowstone
National Park, USA. (See image of 2.6
billion-year-old fossilized remnant.

Bacteria formed microbial mats on land as early as three billion years ago. Fossilized remnants and other biochemical evidence from South Africa suggest that photosynthetic bacteria (primarily blue-green cyanobacteria, that may have included the ancestors of Chroococcidiopsis) may have colonized the wet surface of clay-rich soil during rainy seasons, but were blanketed by aerosol deposits laid down during subsequent dry seasons. Such mats may have formed in surface pools, water edges, and other wet spots on land (Press briefs from the NASA Astrobiology Institute of 2/5/01 and 11/29/00, and from the University of Pennsylvania).

Cyanosite and PSARC -- larger modern and fossilized images

Mix of cyanobacteria from a microbial mat that includes several
filamentous forms, which can also form layered, sediment-clogged
structures called stromatolites -- larger low-tide field image.

Molecular fossils (steranes) of biological lipids (fats from cell membranes) characteristic of eukaryotic organisms that were probably still singled-celled have been found preserved in 2.7-billion-year-old shales from the Pilbara Craton, Australia. Unlike the prokaryotic bacteria (and archaea), the more complex eukaryotes have a nucleus and other organelles within the cell and so are also bigger. Hence, by around Year 1.9 billion, some eukaryotes had developed which would eventually become the ancestors of integrated multi-cellular lifeforms from seaweeds and worms to trees and humans (as discussed below). While not as common as hopanes (the biomarkers of prokaryotes), the trace eukaryotic hydrocarbon biomarkers found in the Archean shales pushed back their geological presence by 500 million to 1 billion years before their known fossil record (Brocks et al, 1999; and Burlingame et al, 1965).

Years 2.1 to 2.6 Billion

Just before this period, some anaerobes mutated to become "aerobic" purple bacteria (proteobacteria) that metabolize molecular oxygen and substances produced by life such as carbohydrates into carbon dioxide and water. Many microbes eventually merged into symbiosis with other microbial types (e.g., acid and heat lovers, swimmers, and oxygen producers and breathers). This was accomplished through ingestion without digestion.

First, some microbes developed a nucleus using cellular membranes to contain their DNA ("eukaryotes"), perhaps through endosymbiosis. Then, some heat and acid resistant archaebacterium (e.g., Thermoplasma) may have merged with free-swimming spirochete-type bacteria, which became flagella or cilia, on a now, free-swimming protist that is easily poisoned by oxygen. Around two billion years ago, however, some of these protists merged with oxygen-breathing purple bacteria, which became mitochondria inside them. Subsequently, some of these aerobic protists merged with photosynthetic bacteria, which became chloroplasts and other plastids, to create free-swimming green algae and the precursors of today's plant cells. As a result, these new microbes -- called protoctists in the Serial Endosymbiosis Theory (SET) of Lynn Margulis -- became quick adapters to new environments and expanded greatly in diversity as well as numbers.

© Wim van Egmond (Photo from Ciliates, used with permission)
Two single-celled protoctists, Euplotes (left) and Stylonychia,
that move with hairlike cilia

Some of the oxygen produced by photosynthetic bacteria was absorbed (oxidized) by iron dissolved in Earth's oceans. This produced an ancient rain of minute, rusty particles to accumulate on ancient ocean floors that is found today as bands of haematite in rock. As molecular oxygen became abundant, a fraction underwent continuous conversion into a tri-atomic form known as ozone (O₃). The ozone rose to form a layer in Earth's atmosphere which helps to protect the planet's carbon-based lifeforms from damage by the Sun's ultraviolet radiation. As photosynthetic bacteria prospered and spread, the concentration of oxygen in air and water became abundant as early as Year 2.24 billion (see update from Bekker et al, 2004). However, anaerobic microbes in many habitats died out in massive numbers, including the climate-warming methanogens.

Earth's primeval atmosphere was also rich in carbon dioxide as well as methane, perhaps 100 times as rich as today. As the Sol was as much as 20 percent less luminous then, this primeval abundance of carbon dioxide and methane initially kept the young cooling Earth warm through a greenhouse effect. While some weather and geologic processes on Earth remove carbon dioxide from the atmosphere, the success of photosynthetic microbes eventually created so much atmospheric oxygen and depleted methane and carbon dioxide levels to such an extent that the greenhouse effect may have become negligible around Year 2.1 Billion chilling the early Earth (Gabrielle Walker, New Scientist, 1999); and Evans and Kirschvink, 1997). As a result, the Earth's surface may have froze mostly or thinly solid through equatorial regions ("Snowball" versus "Slushball" Earth hypotheses) until the level of atmospheric carbon dioxide was boosted to 350 times today's concentration by millions of years of volcanic activity (with a similar increase in methane) and a sudden meltdown occurred -- resulting in an "Acidic Hothouse" (2005 update). Microbial life, however, should have survived in or around cracks in warm ocean seafloors, deep volcanic vents, surface volcanic springs, and other warm niches. A Snowball to Acidic Hothouse swing would have greatly added to already high evolutionary pressures from anaerobic extinctions through genetic isolation of selective survival adaptations and may have led singled-celled eukaryotic organisms to cooperate together physically and form the first multi-cellular lifeforms.

Years 2.6 to 3.6 Billion

© Mike Guiry. Courtesy of the Irish Seaweed Industry Organisation
(Fucus serratus, or "Serrated Wrack," a large multi-cellular protoctist)

Years 3.6 to 4.1 Billion

Earth may have entered a cycle of "Snowball" to "Acidic Hothouse" swings between Years 3.85 and 4.02 billion. This may have occurred because the continents were clustered around the equator, and so a warm Earth would be much more vulnerable to slight cooling trends that trigger a Snowball period. It was not until tectonic movements dispersed the continents north and south that the safety valve provided by chemical weathering kicked back in to restore greater stability. (See a map of the Earth as it looked towards the end of this period about 650 million years ago, when the climate was more like as it is today with mountain glaciers and polar ice. More maps and information can be found at Christopher R. Scotese's PALEOMAP Project.)

David Bottjer, Jun-Yuan Chen, NIGP/AS

Larger image.

600-million-year-old microscopic fossil of a bilateral animal,
Vernanimalcula ("small spring animal"), which evolved after
the long winter of Snowball Earth (more from USC News,
New Scientist, and Pharyngula).

Again, after a massive extinction, intense evolutionary pressure through genetic isolation and selective adaptation may have resulted in a burst of multi-cellular evolution and diversity, leading to the first multi-cellular "animals." Lacking a backbone, these creatures were "invertebrates." Including worms, molluscs, and arthropods (joint-footed animals), invertebrates are among the most successful animals today.

Near the end of this period around 575 million years ago, sediments rich in minerals essential to Earth life (including phosphate, iron, calcium, and bicarbonate ions) may have eroded and washed down from a great range of mountains around the equator -- the Transgondwanan Supermountain -- that was created on the supercontinent Gondwana by the collision of three large continental blocks (Arabia, India, and Antarctica) with the eastern edge of Africa. In combination with rising oxygen levels (and possibly other factors), the increased availability of vital nutrients in the surrounding seas may have kick-started the development of multi-celled Ediacaran fauna, according to Rick Squires's research group. A subsequent collision between Antarctica and Africa raised more mountains and released more sediment from 530 to 510 million years ago may have led to the Cambrian Explosion, when most major groups of animals evolved (including trilobites and bivalves which used abundant calcium to build protective carbonate shells).

Utah Geological Survey,
Millard County, Utah


570-million-year-old Trilobite,
an extinct marine arthropod.

Years 4.1 to 4.6 Billion

Although the Cambrian explosion generated a large number of new phyla of Earth-type life, it actually sputtered out not long after it began so that biodiversity at the family, genus, and species levels was decreasing around 515 million years ago. About 489 million years ago, however, the Ordovician Biodiversification Event began with a second explosion of new life forms. This period was apparently associated with increased meteoric impacts (around 100 times more frequent than today) associated with the break-up in the Main Asteroid Belt of the L-chondrite parent body -- the largest documented asteroid breakup event over the past few billion years. There was a warm, stable climate with dispersed continents surounded by vast warm and shallow seas over continental shelves that provided light, oxygen, and nutrients for life to thrive in, because intense mountain-building also increased erosion and the discharge of eroded nutrients into those seas. In addition, there was intense volcanic activity that generated even more nutrients and yet helped to create more local environments for the evolution of biodiversity (James O'Donoghue, New Scientist, June 11, 2008).

During the Ordovician Period, life also moved onto land. After over three billion years of evolution in the oceans, multi-cellular life -- beginning with green algae, fungi, and plants (mosses, ferns, then vascular and flowering plants) -- began adapting to land habitats by creating a new "hypersea," and adding anomalous shades of green to Earth's coloration. Exploiting habitats that are often or mostly out of water required new symbiotic relationships to contain and move water, including the fusion of some fungi and algae to create lichen in communities with bacteria that survive extreme desiccation on land while breaking down rock into soil, and the association of mycorrhizae fungi and the root tissue of new vascular plants -- culminating in trees that pump water high into the air -- to exchange mineral nutrients (e.g., phosphorus) and usable "fixed" nitrogen from the atmosphere for photosynthetic products. Soon, plant-eating animal life followed (including Arthropods such as the scorpion-like Eurypterids that moved from marine waters into brackish then fresh water -- some species becoming amphibious and emerging onto land for part of their life cycle after becoming capable of breathing in both water and air -- which eventually evolved into insects, and animals with backbones known as "vertebrates" which evolved from Fishes that moved onto land as Amphibians and evolved into Reptiles, Dinosaurs, Birds, and Mammals). Today, some scientists estimate that the biomass of all forms of life that has become successfully adapted to habitats on land has become hundreds to thousands of times greater than that of life in the seas.

NASA (Yucatan Peninsula and Gulf of Mexico)