Now, with this background in Biochemistry we move on to facts and speculations about the origin and development of life on Earth (and by inference, elsewhere in the Universe). This general topic is intimately tied to the theory or concept of Evolution. For our purposes, an understanding of how Evolution works, or even that it is a real process, is not necessary to work through this page. But the ideas behind Evolution are important knowledge, the gist of which is worth your time to become familiar with. So, as an option, before proceeding you are invited to peruse the next page dealing with this subject, accessed by clicking on this highlighted Evolution word; otherwise, defer until the end of this page. On the present page we will consider the progression of life through geologic time but only the evolution of humans will be examined from that perspective.
There is general consensus that the first organic ingredients that became plentiful in oceans ond other energy-rich envIronments associated with water were amino acids and later nucleic acids. RNA probably organized before DNA, and was needed to synthesize the proteins. Both are important in the production of nucleic acids. These constituents had to develop before they could organize into symple prokaryotic cells. The first organisms - bacteria - were single-cells that containing mostly water enclosed in a membrane, with RNA strands and ribosomes as the principal internal organelles.
Evidence is abuilding about the Earth's atmospheric history and its relevance to the appearance of life. The most primitive atmosphere contained Nitrogen, free Hydrogen, some Carbon dioxide, and no Oxygen. In the first two billion years or so, the Sun's energy output was 70-80% of today's radiant release. To keep the early oceans from freezing, some mechanism was needed to maintain proper temperatures. Carbon dioxide - the main Greenhouse gas - could perform part of that function, but its quantity was probably too low (based on the low amounts of FeCO3 or Siderite in the geologic record of early times; likewise, calcium and magnesium Carbonates [limestones] were noticeably rare).
Researchers now think that early on Carbon reacted with Hydrogen atoms to form methane (CH4) which was more efficient than CO2 in absorbing outgoing thermal radiation. A class of living microorganisms - methanogens - could have arisen and flourished for millions of years. The Archaebacteria, the oldest microfossils, were methanogen organisms. As these proliferated, they expelled methane in the atmosphere of the time until that gas reacted with Hydrogen and other constituents to form a "smog" (similar to that on Saturn's Titan) that built up. This in turn would absorb incoming solar irradiation and lead to a reversal of temperatures to the extent that cooling brought about an Ice Age ("Snowball Earth") some 2.3 billion years ago. Thereafter, methanogens never regained their importance as Oxygen slowly built up in the changing atmosphere. This build-up probably resulted from the onset of photosynthesis which produces Oxygen as an end product of the reaction between CO2 and H2O to produce glucose (C6H12O6) plus Oxygen.
There are still questions about how life actually began. The key components - proteins, RNA and DNA - had to precede living cells. Speculation still continues over the mechanisms and circumstances by which the components were first produced. Although not definitively accepted as the actual scenario, an experiment in 1953 by a graduate student, Stanley L. Miller at the University of Chicago, under the tutelage of Nobel Prize winner Harold Urey, is regarded as one of the classic scientific efforts ever in the field of biology. Here is a diagram that depicts the experimental set-up:
Miller and Urey produced a primeval "micro-ocean" in one chamber. Heating it expelled water vapor into a second chamber containing the gases they thought might have existed after the molten Earth cooled to a crust and primitive ocean/atmosphere at a time much hotter than the present. Into the top flask, the water vapor-gases mix was subjected to frequent electrical sparks (to simulate lightning). As days went on, and the condensed mix was sampled and analyzed, sequences of organic molecules were synthesized, as shown here:
Variations of this famed experiment have produced still other organic molecules. The key conclusion it points to is that a reducing, hot atmosphere with compositions similar to the one they used could have generated some of the basic ingredients that later organized into life. Other sources of energy have been proposed. The conditions that prevailed then were probably like those we assign the word "extremophile" to. One plausible alternative is the hot waters around the deep-sea "black smokers" found around oceanic spreading ridges. Carbon escaping from a primitive Earth's mantle would react with other subsurface constituents, especially those in the water, to yield building-block molecules (life today is found around the smokers - apparently produced there - but today's water contains more Oxygen than in primitive earth envIronments [in fact, Oxygen tends to destroy these simpler molecules]). Another view, growing in acceptance, holds that at least some organic molecules were added to Earth after its general melting during bombardment by asteroids/comet (see further comments two pages hence). These extraterrestrial bodies are known to contain various amino acids. An experiment at Lawrence Livermore Laboratory in which a group of amino acids were held in a material subject to high speed impact (from a gun) yielded the surprising result that these acids formed peptide chains, the building blocks of proteins. Thus, life on Earth could have begun internally and/or externally.
On Earth, as stated above the first life was unicellular (microbial, including an abundance of bacteria), followed much later by unicellular plant life which eventually acquired the ability to photosynthesize Carbon compounds using solar energy into monosaccharide carbohydrates, releasing Oxygen as a by-product (a build-up of Oxygen leads to formation of upper atmosphere ozone which, in turn, protects life below from destructive UV radiation). Energy sources that favor life are solar radiation, terrestrial heat, and change of state heat (nuclear decay which supplies much of Earth's heat from the interior may also provide radiation that could synthesize certain organic molecules under the right conditions). (A fourth possibility is gravitational [tidal] energy which might produce life-developing conditions; future exploration of Europa will test this mechanism by seeking life beneath its icy crust). The presence of water and a suitable atmosphere (life on Earth began in a reducing atmosphere but with photosynthesis, Oxygen has increased.
Thus, the frame of reference of any investigations of life elsewhere in the Universe continues to reside in the extensive studies of organic chemistry and biology of organisms dominating the only known place where life's existence is confirmed: our planet. Life on Earth began at least 3.5-3.8 billion years ago (a more precise time has yet to be established, since there is dispute as to the validity of proposed life forms found in ancient rocks of differing ages). Since then the history of life has been increasing complexity and diversity and adaptation of ever more (and changing) envIronments. This chart summarizes this history:
Note: The older span of time in the Archean, from about 3.8 to 4.6 billion years ago, is nowadays given the name "Hadean".
To re-enforce this brief synopsis of the history of life on Earth, we reproduce here this diagram that was placed on the first page of the survey of the basics of Geology shown on page 2-2.
As we shall see on this and the next page, the diversity and complexity of life forms has progressed from simple one-celled plants through a wide range of multi-celled plants and animals. The term "evolution" applies to this. In order to examine and relate this progression, it is necessary to classify and clarify lineages. This is embodied in the term taxonomy. In 1735 Linnaeus first proposed a system of classification that has remained in use as the standard. This diagram summarizes the hierarchy of this system:
Let's now examine some of the life forms themselves, starting with the most primitive. As described above, almost certainly the first living bodies were microscopic in size, being single-celled. Bacteria were the dominant, perhaps the only, major life forms. Below are two modern day examples:
Bacteria are very abundant, as well as primitive. But the record for numbers - at least on Earth - are the viruses known as bacteriophages, which are attached to individual bacteria:
A more advanced modern unicellular animal is the Protist Paramecium, with dual nuclei and numerous tiny "whips" (cilia) for locomotion:Protists first appeared about 900 million years ago.
Familiar to many is the amoeba. This Protist moves by extending parts of its cell as "pseudopods" in certain directions and then pulls the remainder of the cell towards one or more of these protuberances. This photomicrograph shows the Proteus species of Amoeba:
Claims of planktonic microbial life (animals and plants living at or near water surface; free-floating; largely microscopic; utilize photosynthesis in autotrophic or heterotrophic assimilation of foodstuff; in the oceans and lakes planktons are at the base of the food chain) as old as 3.8 b.y. in rocks from Greenland have been made. Here is a modern phytoplankton (microscopic plant)
Generally accepted evidence of the oldest microfossils, cyanobacterial life, found in the 3.465 b.y. Apex Formation of Australia, has been published by J. Wm. Schopf (UCLA) and others. These remain the oldest life of any kind known on EarthIn the field, the Apex Formation is a chert unit that appears thusly:
This next illustration is justly famous as the depiction of the oldest life form known, from the Apex Chert.
The type specimen from the Apex Chert is a cyanobacterium shown here after staining the sample orange.
Here is another set of photos illustrating what has been discovered at the Apex locality:
Microbial life has now been found in the rocks from the Barberton Formation in South Africa, of 3.4 billion year age. The example shown here is in a rock that was emplaced as a glassy lava before crystallizing. It is postulated that this life form actually "fed" on the rock material itself (now recrystallized into a basaltic type).
One of the prevalent life forms (perhaps as far back as 3+ billion years) falls in the general category of cyanobacteria (also known as blue-green algae). Fossil examples from two different ages are shown here:
Cyanobacteria were dominant for at least 2 billion years and some forms still exist today. They produce large amounts of Oxygen by photosynthesis (using sunlight to convert CO2 and H2O to simple sugar and free Oxygen. They played a key role in the transition of the Earth's atmosphere from reducing to a gradual buildup of Oxygen. One of the sedimentary rock types supposedly influenced by bacteria is the Banded Iron Formation (BIF) which occurs worldwide; it forms rich iron ore in Minnesota and Michigan. For an extended period, Iron in water envIronments grabbed the free Oxygen, slowing the buildup of that gas but in time the Iron was depleted and Oxygen then accumulated more rapidly. Here is an example of this BIF rock in which the red is rich in hematite:
Another famous locality containing a variety of ancient life forms is the Gunflint Formation (1.9 b.y. in age) in Minnesota and southern Canada. These are examples of microfossils found in rocks made up of chert from this unit:
Other life forms were fungi and algae. The oldest and most famous of the larger fossils are the stromatolites of Western Australia. Stromatolites are mounds of prokaryotic algae and cyanobacteria. Modern stromatolites occur today along the Australian coast.
These bear resemblance to excavated ancient stromatolites found around Marble Bar in Western Australia dated at 3.45 billion years:
In cross-section these stromatolites have a conspicuous curved layering.
Stromatolites are also found in the Gunflint Formation, described above. Here is an outcrop on Lake Superior: