THE POSSIBLE ORIGINS OF HOMO SAPIENS

A challenge – by Alan Mason –

THE CHALLENGE

I am not surprised that Wikipedia produced a whole raft of information on hominid fossils from Africa.  That is not the point at issue.  One Wednesday evening I asked a very specific question.  “Is there any evidence of ancient (i.e. > 80,000 years ago) Homo sapiens fossils from Africa?”  At the time I did not define “ancient” because I took it for granted that you would understand that I did not mean fossil Homo sapiens from 200 AD or from the Egyptian tombs several millennia BC.

THE THEORETICAL ISSUES

It is generally accepted that the early hominids originated in the savannahs of East Africa and I do not dispute this.  It also seems clear that the genus Homo also came from this region and a number of rather primitive species are recognised.  What is not clear is where the two species, Homo sapiens and Homo neanderthalensis came from. Was it Africa, or as seems more likely the Middle East?  (Now a third species, Homo -?- the Denisovans have been added to the conundrum.)  These are legitimate matters of debate and they can only be resolved by recourse to evidence of various kinds.

FOSSIL EVIDENCE

We only have two kinds of evidence at the moment, ancient fossils and genetical studies.  The latter category includes DNA evidence from fossil bones, and the more general information on DNA from currently existing human races.  It has to be said that this is essentially a geographical argument.  Either (1) Homo sapiens evolved somewhere in Africa, or (2) it originated somewhere else, like the Middle East.  If we find ancient fossil evidence of Homo sapiens in Africa, the first contention is proved.  If we do not find such evidence it does not disprove the first contention, it just means there is no evidence for it.

On the other hand if we regularly find ancient Homo sapiens fossils in Europe, and all over Asia, it is clear that the evidential balance is shifting towards the truth of the second contention.  It does not prove it, but it appears more likely than the first one.   Information from the DNA of fossil bones is relevant to the argument, but it is not clear how DNA from modern human races can offer any help.

MOVEMENTS IN HUMAN EVOLUTION

The fossil evidence suggests that hominids (ape-like pre-humans) originated in the savannahs of East Africa and produced a wide range of different species, most of which were probably short-lived in geological terms.  Extinction is the usual fate of most new species.  The genus, Homo also arose in Africa, and several species have been recognised, but not the two best known species, Homo neanderthalensis and Homo sapiens.

Given that the Genus Homo seems to have been much more successful than the earlier hominids it began to spread more widely and probably contributed to the extinction of the other hominid species because they were in completion for the same kinds of resources.  East Africa is relatively close to the Middle East and any expansion out of Africa would in involve this route.  We are not sure when the modern Mediterranean Sea was created but it was probably after the origin of genus Homo.  Thus a movement out of Africa across a Mediterranean plain was also possible.  It is not worth pursuing the issue of precise routes in the absence of any useful evidence.

The fossil evidence suggests that the species Homo neanderthalensis evolved, probably before H. sapiens, somewhere in the Near East or even eastern Europe, and had spread all over Europe, much of Asia and to a minor extent across the southern Mediterranean.  The very specialised anatomical features were thought to be cold adaptations rather than evidence of a primitive nature.  There was much speculation on whether the Neanderthalers and Sapiens people interbred, and current genetic studies confirm that this was indeed the case if only to a limited degree.

It seems likely that Homo sapiens followed the spread of the Neanderthalers into Europe and Asia competing directly with them for similar resources.  None of this is too surprising as the same kinds of competition occurred among the various species of the genus, Bos, the wild cattle of Europe and Asia, or the different species of deer.

The arrival of the species, Homo sapiens appears to have been such a major advance that it quickly underwent adaptive radiation.  This is a common feature of evolution, and it means, briefly, that a wide variety of new forms are produced, more or less contemporaneously,

THE END OF THE NEANDERTHALERS

It is possible that the Neanderthalers underwent adaptive radiation, but we would need far better evidence to establish the truth of this.  At present the limited collections of bones cannot successfully provide this.

The reason for the disappearance of the Neanderthalers still remains a mystery, but it has to be repeated that extinction is the normal fate of most species.  The Neanderthalers live on in the gene pool of modern humans probably because these genes are useful to us.  In just the same way the dinosaurs live on today, “Look, there they go.”, as a flock of starlings swoops across the sky.  The valuable gene pool of the dinosaurs lives on in modern birds.

The wholly new discoveries of a third advanced species of the genus Homo in Siberia fits quite well into the scenario just described.  It is perfectly possible for hominids to have left Africa and radiated into three new species of Homo; a western one, (H. neanderthalensis) a south-central one, (H. sapiens) and a north-eastern one, (Homo –?) the “Denisovans”.

We return now to Homo sapiens and its adaptive radiation.

SUB SPECIFIC VARIATION

Almost every species of animal or plant that has been studied closely, reveals the existence of distinct groups below the species level.  Sometimes these are called sub-species, sometimes varieties, and sometimes races.  The issue of interbreeding does not arise, because if the members of different subgroups are part of one species, by definition, they must be able to interbreed to produce fertile[1] offspring.

The variation in terminology is a clue to the problem of selecting a suitable name and a set of characters.  What happens in practice is taxonomists select a set of criteria which they think are important, like anatomical structure, appearance in colour or pattern, biochemical indicators, and behavioural patterns.

It is important to remember that evolution is a dynamic process and consequently new species are continually evolving and older species are becoming extinct.  Thus sub-species, varieties, and races are potential new species.  Only potential however; success is not guaranteed.

SOCIO-POLITICAL ISSUES

Wouldn’t it be strange if the human species was the only one on earth without recognisable groups at the sub-specific level?  In the early part of the 20 th century (20s to 50s) anthropologists and comparative anatomists tried a number of different classifications of humanity and the issues were debated freely.  Some systems were rejected on technical grounds of evidence and other systems were seen to be more satisfactory on evidential grounds.

However, by the 60s all this work began to go underground.  Political correctness had arrived.  Because some nations had embraced racism as a political creed and had oppressed or murdered their citizens it became impossible to have a neutral or evidence-based discussion on the existence of human races.  Any scientist trying to obtain funds, or even an academic post in this area of research would find himself frozen out.  Hence, through ignorant, and mis-guided political interference a whole area of legitimate scientific enquiry was almost closed off.  I say, almost, because in a relatively free country the truth will often manage to find its way in through chinks in the wall erected around it.

“PRIMITIVE” OR “ADVANCED”?

When biologists use terms like “primitive” or “advanced” about biological species it depends a lot on actually knowing something about the evidence.  For example, botanists call Magnolias “primitive” flowering plants but grasses are “advanced” ones.  How can this be?

Magnolias have large flowers whose petals and other parts are very similar to non-flower parts like leaves.  It seems as though they have not evolved far.  Fossils from 60 m y a show flowers very similar to modern Magnolias.

Most people think of grasses as very simple and probably primitive plants.  Botanists know that the flower parts are small and highly evolved in structure and function.  Grasses do not appear in the fossil record until quite late in the history of their group.  Their very simplicity has given them an enormous evolutionary advantage and this is why they are one of most widespread and successful groups of plants.  I use this example because things are not always what they seem to the uninformed.

ADAPTIVE RADIATION IN HUMAN RACES

I don’t want to get embroiled in the debates on the technicalities of human classification so I shall stick to some very basic descriptions of only three major sub groups.  There are, of course, several other significant groups, like the Australoids, and Polynesians, but these do not affect the main argument.

Most anthropologists had come to the conclusion that the Indo European group was probably early and primitive.  They had a set of rather unspecialised features which, in other animal groups suggests they had not evolved far.  Their general features are loose, wavy hair in various colours, male beards, tall stature and skin colour ranging from white to brown.  Other more technical issues are skull shape, blood group frequencies etc.

By contrast, the Asian group was seen as more advanced and specialised, probably to cope with icy conditions in the north.  Typically, they were a smaller and more compact people to reduce their heat losses.  Their hair was long, and black with an oval cross section under the microscope, unlike the round Indo European type. Male beards were scanty.  Skin colour varied from pale to brown, and head shapes were broad.  The classic epicanthic fold of the eyes and the fat-filled cheeks were seen as adaptations to persistent cold winds.

The African racial type was thought to be also highly evolved to cope with the great heat of the sun.  They were often tall, with long heels, and a linear build to enhance heat losses.  Their black hair had the typical tight curl which created a dense rigid mat over the skull.  Male beards were scanty and skin colour varied from deep black to light brown.  Again, technical details included long skull shape, and specific blood group frequencies.

Now, how were these variations related to evolution and movements of populations?  The picture seemed to be that Homo sapiens had appeared, probably in the Middle to Near East and had radiated out into several physical types based upon climatic factors.  The Indo European group spread into Europe and western Asia.  In the north and west pale skin colour and fair to red hair evolved in response to low sunlight levels.  In the east brown skin colour was dominant in sunnier climates.

The Asian group, although specialised for the cold windy conditions of the north, spread southwards and became adapted to the sub-tropical regions, so that southern Asians evolved browner skins.  Some migrated into North America and underwent further adaptation as what are now termed, “Native Americans”.

The African group returned to Africa, quickly out-competing and exterminating all the other hominids and spreading far wider than the original savannahs of the east.  Widely different physical types evolved so that the Africans of the west were very stocky, big-boned, and very black with broad flattened noses and running to fat.  By contrast, in the east the people were slender, and brown skinned with long narrow noses.  Several pygmy groups evolved like the Bushmen of the Kalahari Desert.

It is likely that the increasing information from the DNA of present human races will enable us to tie the different anatomical variations, like hair type, for example, to particular collections of genes.  Eventually, we may be able to precisely chart the adaptive radiation described earlier.  Finally, we may be able to “work back into the past” and establish a time frame for the different changes that have occurred in human evolution.

As science frequently comes up with new techniques undreamed of by previous generations, so it may be possible to establish with certainty exactly where and when Homo sapiens arose.

[1] Note that correct term is fertile, not viable.  Horse/donkey crosses produce mules which are viable, i.e. capable of life, but they are not fertile, in that they cannot reproduce.

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One Response to THE POSSIBLE ORIGINS OF HOMO SAPIENS

  1. Deskarati says:

    Further to the challenge regarding the possible origins of Homo Sapiens, Deskarati found this article in Science Daily from 2005 which, on the face of it, seems to show fossils long before the 80,000 year ‘ancient’ date. Unfortunately, we are not knowledgeable enough on the subject to substantiate this article or confident that it has not been disputed in the years since publication. Still we thought it would be interesting to move the debate along – Dekarati

    ScienceDaily (Feb. 28, 2005) – When the bones of two early humans were found in 1967 near Kibish, Ethiopia, they were thought to be 130,000 years old. A few years ago, researchers found 154,000- to 160,000-year-old human bones at Herto, Ethiopia. Now, a new study of the 1967 fossil site indicates the earliest known members of our species, Homo sapiens, roamed Africa about 195,000 years ago.
    “It pushes back the beginning of anatomically modern humans,” says geologist Frank Brown, a co-author of the study and dean of the University of Utah’s College of Mines and Earth Sciences.
    The journal Nature is publishing the study in its Thursday Feb. 17, 2005, issue. Brown conducted the research with geologist and geochronologist Ian McDougall of Australian National University in Canberra, and anthropologist John Fleagle of New York state’s Stony Brook University.
    The researchers dated mineral crystals in volcanic ash layers above and below layers of river sediments that contain the early human bones. They conclude the fossils are much older than a 104,000-year-old volcanic layer and very close in age to a 196,000-year-old layer, says Brown.
    “These are the oldest well-dated fossils of modern humans (Homo sapiens) currently known anywhere in the world,” the scientists say in a summary of the study.
    Significance of an Earlier Emergence of Homo sapiens
    Brown says that pushing the emergence of Homo sapiens from about 160,000 years ago back to about 195,000 years ago “is significant because the cultural aspects of humanity in most cases appear much later in the record – only 50,000 years ago – which would mean 150,000 years of Homo sapiens without cultural stuff, such as evidence of eating fish, of harpoons, anything to do with music (flutes and that sort of thing), needles, even tools. This stuff all comes in very late, except for stone knife blades, which appeared between 50,000 and 200,000 years ago, depending on whom you believe.”
    Fleagle adds: “There is a huge debate in the archeological literature regarding the first appearance of modern aspects of behavior such as bone carving for religious reasons, or tools (harpoons and things), ornamentation (bead jewelry and such), drawn images, arrowheads. They only appear as a coherent package about 50,000 years ago, and the first modern humans that left Africa between 50,000 and 40,000 years ago seem to have had the full set. As modern human anatomy is documented at earlier and earlier sites, it becomes evident that there was a great time gap between the appearance of the modern skeleton and ‘modern behavior.'”
    The study moves the date of human skulls found in Ethiopia’s Kibish rock formation in 1967 back from 130,000 years to a newly determined date of 195,000 years ago, give or take 5,000 years. Fossils from an individual known as Omo I look like bones of modern humans, but other bones are from a more primitive cousin named Omo II.
    In addition to the cultural question, the earlier date for humanity’s emergence is important for other reasons.
    “First, it makes the dates in the fossil record almost exactly concordant with the dates suggested by genetic studies for the origin of our species,” Fleagle says. “Second, it places the first appearance of modern Homo sapiens in Africa many more thousands of years before our species appears on any other continent. It lengthens that gap. … Finally, the similar dating of the two skulls indicates that when modern humans first appeared there were other contemporary populations [Omo II] that were less modern.”
    The study was funded by the National Science Foundation, the L. S. B. Leakey Foundation, the National Geographic Society and the Australian National University.
    Modern Homo in the Valley of the Omo
    Richard Leakey and his team of paleontologists traveled in 1967 to the Kibish Formation along the Omo River in southernmost Ethiopia, near the town of Kibish. They found the skull (minus the face) and partial skeleton (parts of arms, legs, feet and the pelvis) of Omo I, and the top and back of the skull of Omo II. Brown was not part of the 1967 expedition, but was working nearby and got to look at the site and the fossils.
    “Anthropologists said they looked very different in their evolutionary status,” Brown recalls. “Omo I appeared to be essentially modern Homo sapiens, and Omo II appeared to be more primitive.”
    In 1967, the fossils were dated as being 130,000 years old, although the scientists doubted the accuracy of their dating technique, which was based on the decay of uranium-238 to thorium-238 in oyster shells from a rock layer near the skulls.
    Fleagle says no scientist has been bold enough to suggest Omo II is anything other than Homo sapiens, and that “quite often at the time of major events in evolution, one finds an increase in morphological [anatomical] diversity.” Now that the new study confirms Omo I and Omo II are the same age – living within a few hundred years of each other about 195,000 years ago – some anthropologist suggest “maybe it [Omo II] isn’t so primitive after all,” Brown says.
    McDougall, Brown and Fleagle and researchers from other universities returned to Kibish in 1999, 2001, 2002 and 2003. They identified sites where Omo I and Omo II were found in 1967, and obtained more of Omo I, including part of the femur (upper leg bone) that fit a piece found in 1967. They also found animal fossils and stone tools, and studied local geology. The Nature study includes initial results from those expeditions.
    The fossil record of human ancestors may go back 6 million years or more, and the genus Homo arose at least 1.8 million years ago when australopithecines evolved into human ancestors known as Homo habilis. Brown says the fossil record of humans is poor from 100,000 to 500,000 years ago, so Omo I is significant because it now is well dated.
    Dating the Dawn of Humanity
    Both Omo I and Omo II were buried in the lowermost portion or “member” of the Kibish Formation, a series of annual flood sediments laid down rapidly by the ancient Omo River on the delta where it once entered Lake Turkana. Lake levels now are much lower, and the river enters the lake about 60 miles (100 kilometers) south of Kibish.
    The 330-foot-thick (100-meter-thick) formation is divided into at least four members, with each of the four sets of layers separated from the other by an “unconformity,” which represents a period of time when rock eroded away instead of being deposited. For example, the lowermost Kibish I member was deposited in layers as the Omo River flooded each year. After thousands of years, rainfall diminished, lake levels dropped, and the upper part of Kibish I eroded away. Later, the lake rose and deposition resumed to create layers of Kibish member II.
    Interspersed among the river sediments are occasional layers of volcanic ash from ancient eruptions of nearby volcanoes. Some ash layers contain chunks of pumice, which in turn contain feldspar mineral crystals. Feldspar has small amounts of radioactive potassium-40, which decays into argon-40 gas at a known rate. The gas, trapped inside feldspar crystals, allows scientists to date the feldspar and the pumice and ash encasing it.
    Brown says potassium-argon dating shows that a layer of ash no more than 10 feet (3 meters) below Omo I’s and Omo II’s burial place is 196,000 years old, give or take 2,000 years. Another layer is 104,000 years old. It is almost 160 feet (50 meters) above the layer that yielded the Omo humans. The unconformities represent periods of time when rock was eroded, so the fossils must be much older than the 104,000-year-old layer and close in age to the 196,000-year-old layer, Brown says.
    The clinching evidence, he says, comes from sapropels, which are dark rock layers on the Mediterranean seafloor that were deposited when floods of fresh water poured out of the Nile River during rainy times. The Blue Nile and White Nile tributaries share a drainage divide with the Omo River. During ancient wet periods, monsoons on the Ethiopian highlands sent annual floods surging down the Nile system, causing sapropels to form on the seafloor, and sent floods down the Omo, making Lake Turkana rise and depositing Kibish Formation sediments on the river’s ancient delta. (During dry periods, Lake Turkana was smaller, flood sediments were deposited farther south and rocks at Kibish were eroded.)
    No other sediments on land have been found to record wet and dry periods that correlate so well with the same climate pattern in ocean sediments, Brown says. The new study found that the “members” – or groups of rock layers – of the Kibish formation were laid down at the same time as the Mediterranean sapropels. In particular, the volcanic layer right beneath Omo I and II dates to 196,000 years ago by potassium-argon dating, and it corresponds almost perfectly to a sapropel layer previously dated as 195,000 years old, Brown says.
    “It is pretty conclusive,” says Brown, who disputes any contention that the fossils might be closer to 104,000 years old.

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