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. 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 holds that at least some organic molecules were added to Earth after its general melting during bombardment by asteroids/comet. 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. 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:
From Raven & Johnson, Biology, 6th Ed., McGraw-Hill Higher Education.
From Raven & Johnson, Biology, 6th Ed., McGraw-Hill Higher Education.
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: The first eukaryotic life forms may be as old as 2 billion years ago. Multicellular algae were common by about 1.2 b.y. ago; wormlike creatures (possibly animals) had appeared by then. Best known among the Eucaryotic algae is Grypania spiralis, found in ancient rocks in Michigan and in Australia. This fossil is preserved because it formed simple shells:
Life continued in this primitive state until about a billion and a half years ago when photosynthesis became a common process that helped plant life flourish and released Oxygen. The first multicelled plant is similar to modern brown algae; pieces of this have been found in Chinese rocks dating to about 1.88 billion years ago.
In the Proterozoic (2.5 billion years to 540 million years), life forms slowly changed, with eukaryotic (nucleus) single-celled phylla becoming more diverse. Protozoa (primitive forms) were joined by Metazoa (more advanced) about 1 billion years ago. The first precursors to animals were ancestors to the sponges. Here is an example:
The oldest multicellular animal yet discovered is a sponge(like) fossil from rocks in Oman. These date at approximately 710 million years. No actual fossil form was recovered (sponges only have soft parts) but instead a distinctive chemical was found in the carbonate rocks. The telltale sign present in the rocks is a fatty chemical called 24-isopropylcholestane, which scientists have found only in the skeletal structures of demosponges, the most common member of the sponge family
Claims of a unicellular fossil trace in Australian rocks that are about 1.2 billion years old are still in dispute as the evidence could be the result of inorganic activity. Thiomargarita is the name given to markings in Chinese rocks about 600 million years old that have been interpreted as eggs or embryos, and by others as bacteria
The greatest explosion of life in earth history took place about 600 to 500 million years (late Proterozoic into Cambrian) ago with the appearance of distinctive and diverse animal forms. One significant marker was the first appearance of animals with bilateral symmentry at least 600 million years ago. These animals were very small, generally microscopic, and usually found in shales; the samples had to be thin-sectioned by guesswork to find the tiny objects which were preserved soft parts. Representative of these is Vernanimalcula, found in China:
Some taxonomists place the appearance of bilateralism even earlier - one school maintains about a billion years ago. The common ancestor to all animals has been named Unilateria (no such fossils have been found). A general scheme for the evolutionary chain of animals follows this lineage:
A significant upswing in the numbers and diversity of life marks the last 200 million years of the Proterozoic (often referred to as the "Neoproterozoic"). This is a time-stratigraphic chart that introduces Periods that will now be discussed:
Two of the most common guide fossils to Vendian life are shown here as actual specimens.
By the opening of the Cambrian, many forms of invertebrate life had developed (mainly for protection) external carapaces or coverings that, after death, survive as fossil shells. A survey of Cambrian stratigraphy indicates a dramatic increase in diverse marine life forms, so much so that this abundance of living creatures has been referred to as "The Cambrian Explosion".
Around 535 m.y. ago, in the early Cambrian, a soft parts assemblage of fossil forms has been found in the Chengjiang Formation in Yunnan Province of southern China. Here are typical fossils;
This is an artist's conception of typical animal and plant life in the shallow sea in which the Burgess shale was deposited:
Some of the typical life forms in the Burgess shale, each fascinating in its own right, are shown below:
An excellent review of early life on Earth is available at this web site maintained by the University of Munster (unfortunately, many of the links no longer are active).
A second peak time in the abundance of shell-surviving life forms was in the Upper Ordovician (by this time also, the first larger vertebrates, fossil fish, had appeared). Below are two illustrations: the first, an artist' conception of marine invertebrate life in the late Ordovician; the second, a typical slab of Ordovician limestone (from Indiana) containing the fossil types listed in its caption:
The oldest known vertebrate life may be a tiny fish (8 cm in length) called Anatoleptis, of very late Cambrian age (510 million years; younger specimens (470 m/y.) have been found in Scandanavian rocks. The photo below is a microscope view of scales from this fish, whose remains have been found in Wyoming and other parts of North America:
Almost nothing is known about life dwelling mainly on land. A small millipede found in rocks in Scotland, dated at 428 million years ago, is a candidate for earliest terrestrial life:
There have been several extended time spans (but occuring mainly within single Periods) when major spurts in evolution have witnessed appearances of whole new classes of animal life forms. The Cambrian Explosion described above is the prime example. In the Devonian, the first amphibians (land-dwelling animals), insects, and land trees have been found in the fossil record. A "missing link" (375 million years old) between fish and amphibians was found on Ellesmere Island in the Canadian Arctic. Known as Tiktaalik roseae, it has a crocodilelike head, was 3 meters long, had fins with articulation that foretold jointed legs, and probably could wiggle like a seal on land when not in the sea. Here it is:
We have alluded to the huge increase in animal life since the "Cambrian Explosion" witnessed in the Burgess Shale. This is a good place in the narrative to summarize animal life since the end of the Precambrian.
Although the most ancient life was a mix of single celled plants and animals, we have not touched upon the more advanced forms of life since the Precambrian. A major and vital evolutionary spurt was when marine plant life adapted to living on the land. This began about 480 million years ago. These next two diagrams show the history of land plants:
The illustration below show modern brown algae which were likely progenitors of the land plants.
By the Devonian land plants, which had started as ferns, club mosses, horsetails, and liverworts (all of these still exist), had established vascular systems, acquired the ability to root in the soil, and were able to propagate using seeds. One of the first giant trees is Archaeopteris, shown in this drawing:
A typical plant in this early stage of evolution is Cooksonia, shown here as a fossil frong:
The best known locality for early land plant fossils is the Rhynie chert deposits of Scotland:
This drawing below is a reconstruction of a typical Devonian forest:
The Carboniferous forests are among the most diverse in geologic history. Their decay in swamps has produced coal beds world wide.
Among the giant plants in the Carboniferous forest shown in this panel were Cordaites, an early relative of conifers; Calamites, a bushy horsetail; Medullosa,a seed fern (a plant with seeds and fern-like leaves); Psaronius, a tree fern; and Paralycopodites and Lepidophloios, lycopsids (scaly, pole-like trees with cones). Lepidophloios could grow to 40 m (132 ft), but most of today’s lycopsids, known as quillworts and club mosses, grow only a few centimeters high.
Illustration by Mary Parrish © Smithsonian Institution
The illustration below shows a cluster of leaves of Pennsylvanian age:
The subject of land plants has been glossed over in the above paragraphs. For those seeking more information, consult the Review of plants and the Earliest land plants web sites.
As mentioned in Section 18 and elsewhere, some of the major advances or disappearances in life forms are being attributed to worldwide effects of large impacts. But other causes of these compressed spurts or declines are probably involved in other cases: extreme volcanism; dramatic climate changes; continental splitting, are suggested. These may be manifestations of the "Punctuated Equilibrium" mode of evolutions proposed by Stephen Jay Gould and others. A rather fanciful panorama of life forms (Kingdoms, Phylla, Families, etc.) from the Late Precambrian to the Present is shown in this mural that is found on the Humboldt State University campus. On the left, pre-Paleozoic animals with soft and hard parts give way to the Invertebrates of the Early Paleozoic, the first fish (Ordovician; sharks in the Devonian), then Amphibians that appeared at about the same time (Mississipian) that land plants took root, with the first reptiles and dinosaurs near the end of the Paleozoic (about the time the first extensive forests spread in the Pennsylvanian), reptiles and small mammals in the Mesozoic, along with the first birds, and finally a dominance of mammals, flowering plants, and widespread forests in the Cenozoic. Missing from the far right of this panel is the story of the hominids, which includes today's mankind. Ancestors to the hominids have been found as far back as 4.5 million years (older animals that may be links are still controversial). This subject is far too involved for any extended treatment in this Tutorial, but the following synopsis touches upon many of the key ideas. The study of ancient precursors to modern humans, is called Paleoanthropology; see this Wikipedia site for a review of mankind's genealogy.
A good, quick overview that carries back to the beginnings of life 4 billion years ago but gives some emphasis to the appearance of humans is found in the Wikipedia Timeline of Human Evolution webpage. For this present page, these three diagrams are helpful. The first is a chart that gives a general time line through the Cenozoic and into the end of the Mesozoic; the second shows one version of primate evolution; the third a variation of this with hominids included and broadly spelled out:
To see entire panel, scroll bottom bar to the right
Classification of the Primates show two broad groupings: 1. The Prosimians, made up of Lemurs, and 2. The Anthropoids, that includes Old and New World monkeys, the Apes, the Chipanzees, and Humans. Of the Anthropoids, there are two major divergences over the last 30 million years. One line includes Old World monkeys, the other contains several branches of which one is the hominids. While the closest tie between Man and other similar animals is said by many to be with the Gorilla or other Apes, the Chimpanzees are also related (98% of the thousands of DNA genes in the genomes of Man and Chimp are identical and located in the same relative positions).
Much publicity has been given to an announcement in May of 2009 of what is called by some as the "missing link" ancestor of both Primate Groups. A nearly complete primate fossil was found in the 1980s in the Messel shale quarry near Frankfurt, Germany. The rocks there are 47 million years in age, placing the many fossil types at Messel in the Eocene.The primate's formal name is Darwinius messelae, but it is called "Ida" (after the daughter of the scientist who led the recent research on it). These two illustrations are pertinent:
The fossil appears to be a transitional link between the Prosimians and the Anthropoids. It has some lemurlike qualities but lacks certain features of that group while showing some bones that link it to the Anthropoids. Of particular import is the opposable thumb in the hand; this is what is characteristic of primates among mammals and a trait that facilitated the ability of humans to evolve into civilized Man.
We will now concentrate on human evolution, as determined mainly from the fossil record. As a generality, the genus "Homo" is associated with modern humans and direct ancestors whereas the genus "Australopithecus" and others refer to pre-humans. Two diagrams are introduced here but the second is a repeat of a diagram shown on page 19-2a:
Part of this progression can be displayed in terms of an evolutionary pattern that depends on the principal evidence - interpretation of skeletal remains, in this case reconstructed heads.
The diagram below shows the skull differences in a different rendition:
This next diagram was taken off the Internet; it is a reproduction of an August 1999 feature article in Time Magazine. It is worth examining before we add comments:
Fossil bones of various hominids have been found on all Old World continents, as evident in this map:
To most Americans, the Leakey family are the best known paleoanthropologists. Mary and Louis Leakey (and now their son, Richard Leakey) established the first lineage for the ancestors to genus Homo with their discoveries in Olduvai Gorge in Tanzania. They along, with other scientists, have visited several African countries, with usually more than one locality in each country, with finds that are pictured in this diagram:
The "holy grail" for paleoanthropologists is the so-called "missing link" - the skeleton that can be considered the "starting point" for the hominids. This will likely be somewhere in Africa. Several candidates have now been found, but the experts do not agree on whether these are actually ancestral or are some offshoot in the evolutionary pathway back to the first creature than can be definitively identified as the starting point. One such candidate is named Sahelanthropus tschadensis, and is about 6 to 7 million years old.
The term "hominid" goes beyond what we picture as humans - it is applied to two-legged animals that are bipedal (walk upright most of the time), which includes some of the apes. New fossil finds (generally parts of skulls and leg bones) in recent years suggest that two species are important progenitors to human lineage: Orrorin tugenensis (~6 million years ago) and Ardipithecus ramidus (4.5 m.y). Ardipithecus ramidus was discovered in Ethiopia in 1994 and is now considered to be the best documented precursor to the main line of hominids. Evidence from skeletons about half complete suggest that "Ardi" walked upright but also moved about trees. The next two illustrations show first the best example of a A. ramidus skull, next a reconstructed full skull and then a reconstructed complete skeleton from which a drawing of Ardipithecus ramidus has been produced:
An informative discussion about these early hominids is found on this Wikipedia website.
The genus Australopithecus first appeared about 4.2 m.y. ago. A direct ancestor of the human race is generally agreed to be Australopithecus afarensis. The most famous of all paleoanthropological finds is "Lucy", of this species, found in northern Ethiopia (by Donald Johanson and his colleagues) in volcanic deposits dated at 3.2 million years. Here is her partial skeleton:
The skull of Australopithecus afarensis is distinctive, in part because of its rather close resemblance to genus Homo:
No complete Australopithecus skeleton has yet been found, but parts from different individuals help to fill the gaps. This illustration shows a reconstructed complete Australopithecus skeleton compared with modern Homo sapiens:
The genus Homo goes back to just over 2.3 million years; this branch evolved from the australopithecines (A. in the diagram) that extend back at least 4+ million years. Here is a skull of Homo habilis:
Homo habilis used crude tools - stones for breaking bones, and possibly cutting tools made by chipping flint. There is debate as to whether H. habilis was a hunter (killed animals for food) or a scavenger (fed off of kills done by other animals). The species lived mostly on the ground (in the African savannah [grasslands]) but took to the trees for safety.
Homo erectus is the immediate ancestor to modern Man. Homo erectus, which lived from about 1.8 million to 300000 years ago, was tall (more slender, and often above 6 feet in height). Homo erectus may have been the first homonid to develop a crude language. Bone remains of H. erectus have been found in Europe and Asia (Peking Man), even to Indonesia (Java Man). Just where H. erectus originated is not yet established; Africa is one possibility. Here is an artist's rendering of a group of H. erectus primitives hunting in the jungle:
The last few paragraphs have introduced a lot of new information. A good overview/review is found at this website:Hominid species.
What would become modern humans - Homo sapiens - appeared about 200,000 years ago. Prior to that a form called archaic Homo sapiens co-existed with later H. erectus. H. sapiens probably evolved from H. heidelbergensis (as did H. neanderthalis). We show below a famous painting that represents the major "players" in Man's ancestry. Read the caption for identity of each individual:
There are two schools of thought about the geographic origins of H. sapiens. One, the Multiregional group, considers that various strains of H. sapiens developed from Homo erectus in several regions of Asia and perhaps Europe. In this view, Homo neandertalensis (which settled mainly in Europe and the Near East) began in the western part of Asia and moved into Europe. If the Multiregional hypothesis proves valid, one or more archaic H. sapiens are the direct result. The second school is the Out-of-Africa group. Currently, this is now the more favored hypothesis.
In the last decade, mitochondrial DNA analysis (identifying the genes and breaking the organizational code to establish their sequence;) has led to strong evidence that H. sapiens originated in the savannah of eastern Africa. Tracing the genetics back in time has identified a baseline DNA case represented by a single individual, whimsically named "Eve" by paleobiogeneticists. Populations were small, not well intermixed, and adapted to warm climates. Based on evolutionary rates, Eve may be as old as 200000 years, but since no fossils of that age have yet been found, this progenitor may in fact be younger.
Thus, it appears that Homo sapiens began in Africa, from the Homo erectus progenitors that had settled there, and remained on the continent for about 150000 years. Some 40 genetic units lived there but were largely isolated from one another. One group is represented today by the "Bushmen" but most of the other units have died out or dispersed. There is some recent evidence that suggests H. sapiens almost died out, probably because of severe drought.
Mitochondrial DNA evidence has allowed tracing of these African units both within that continent and more recent dispersion. About 60000 years ago, some of the African units left that continent (again, drought is suspected as the cause of a need to find more find and amenable environments) and entered the Middle East and western Asia. These spread further reaching eastern Asia and Australia about 50000 years ago. Some clans moved westward into Europe about 35000 years ago (where Cro-Magnon [of cave-painting fame] co-existed for a while with the Neanderthals. (The Neanderthals, stocky and robust, were at the time better adapted to live in a climate then dominated by glacial conditions; the mental picture of a "stupid" subhuman developed when an anthropologist released [about a hundred years ago] a picture of a Neanderthal that portrayed this species as a brutish caveman-like cousin to the apes has been debunked by DNA studies (the brain of Neaderthals is actually about 20% larger than H. sapiens); the smarter Cro-Magnons may have been responsible for the demise of the Neanderthals, either by interbreeding or more likely by necine force.) Others moved across the Bering Strait into North and then South America, with notable migration into these continents about 15000 years ago (there is evidence of even earlier Man in parts of western N. America). During these last 60000 years of Exodus, genetic variations gave rise to the races and strains we observe today. A major factor in development of the races was the need to adapt to changing environments that contained different exposure rates to solar Ultraviolet rays; this UV is moderated by Vitamin D that controls the adaptation of body pigments through mutations. The original race within H. sapiens is closely related to the Negro of today. Most readers of this Tutorial know a bit about the co-existence of Neanderhals and Cro-Magnons. The latter is one of the groups of Early Modern Humans (EMH) that dispersed worldwide. The Neanderthals were once considered a subspecies of EMH but most paleoanthropologists now consider them to be a separate species (see entry on Neanderthals at this Wikipedia site).
Comparisons of the skeletons of the two homonids show distinct differences:
The difference between a Neanderthal and a 21st Century Caucasian man is suggested by this intriguing image:
Cave wall painting is often cited as the clear sign of a distinct culture (for a summary of this art, move to this Wikepedia site. The earliest attempts at art may be markings in caves on the Nullarbor plains of South Australia. Wall paintings in southern European caves may be as old as 35000 years. This famous example, dated at 16000 years ago, is on a wall in a cave near Lascaux in southern France:
Anthropologists classify hominid pre-history using these terms: Stone age (subdivided into Paleolithic - a span over beginning more than a million years in which several Homo species used crude rock tools; Mesolithic - about 12000 to 7000 years ago; and Neolithic - 7000 to about 5500), the Bronze Age (5500 to 3200 years ago), and the Iron Age (starting about 3200 years ago). There is no evidence for when language - the use of spoken sounds to communicate ideas - began but it may have been earlier than 25000 years ago. Crop cultivations may have started about 12000 years ago.
Early civilizations (defined as having established large groups with distinctive cultures that are recorded as written histories) began in Mesopotamia, the Nile valley of Egypt, the Indus Valley region of modern Pakistan, in the Huang He (Yellow River) valley of China, and on the island of Crete in the Aegean Sea. These peoples built cities, created writing systems, learned to make pottery and use metals, domesticated animals, and created complex social structures with class systems. A timeline for these civilizations:
After this overview of the nature of life and its evolutionary progress, we need to expand our thoughts on how life may actually have originated in space, beyond just the Earth. This page is continued as page 20-12a, reached through the Next button.
