The Hominid Succession
from the skeptic, Vol 19, No. 4
Helen Lawrence has completed a BA in which she studied Anthropology.
Creationist myths were a legitimate approximation to the truth at a time when an answer to where we came from was not known. This is no longer the case. We do know, and in some detail. We have been bred from proto-human forms. About 8 million years ago or somewhat less, a common ancestor gave rise to the chimpanzee (Pan) and us. The chimpanzee is the ape most closely related, with only one percent of molecular difference between our genus and theirs. Evidence of evolution of species has become apparent on many scientific fronts, but nowhere can it be more dramatically demonstrated than in the human fossil record. In the Australian Museum in Sydney, a long line of skulls shows the progression through proto-human to human form, known collectively as hominids. In the hominids, brain size has increasingly outstripped body size in relative proportion. The museum exhibit is visual evidence that there is no break in sequence, no possibility of separating ourselves as something special. Homo sapiens is simply part of the hominid succession.
It is difficult for our species to grasp the meaning of deep time, of one species changing to become another. Outside the pages of science fiction we do not expect a chimpanzee to give birth to a human being, and we know this has never happened. Instead, there has been gradual change in the skeleton, brain and external features such as body hair, along with almost no change in internal organs. In the eighteenth century, Peter Camper dissected an orangutan and was rather alarmed by this! New species are formed by an event causing isolation from the parent group. Differences accumulate, to the extent when it seems reasonable to define a new species. Humanity, as part of the living world, has followed this pattern. I intend to show, in brief, how these forms have succeeded one another.
There has been a regular cascade of fossil evidence in the last two decades. So many of our ancestors have been discovered that it takes a body of scientists to describe them all. Not only can hominid bones be identified, but their individual brain volume can be estimated along with their height, weight, walking mechanics, arm and leg length and hand function. These techniques are augmented by the study of stone and bone tools and the things that hominids made with them; the study of animal bones - what the hominids were eating; and by climatology and palaeobotany - what the climate was like and what grew under those conditions. All of this information is backed up by geologists and dating experts. Geologists are widely accepted as knowing a lot about rocks, but dating methods are very technical and harder to grasp. Every scientific discipline is prone to error and must be open to review, but to say that all dating is suspect is layman arrogance and a kind of insanity. Basically there are four dating methods:
a) dating using radioactive methods, eg radiocarbon dating of charcoal, decay of uranium to daughter isotopes;
b) dating by release of electrons trapped over a time period, eg thermoluminescence;
c) dating using polar magnetic reversal which happens at known times and which aligns particles in rock north/south, reversing them when there is a polar reversal;
d) dating using chemical changes, eg breakdown of amino-acids; change in the nitrogen and fluorine content in bones. (As bones age, nitrogen is lost but fluorine is gained, absorbed from ground water. These two measurable changes can be used to check one another).
Since one method can be checked by several others, verification can be carried out. For instance, when a hominid skeleton is found, the bones themselves are dated if possible, but the actual rock strata where they were found is also submitted to dating, sometimes by several of the above methods. The strata may contain animal bones which can be dated by a known stage in their evolution. Added to all these checks is the information from the geneticists. Recently, a sample of DNA in the femur of the first found Neandertal has been examined. It differs from the human version which makes Neandertals almost certainly not close relatives of ours, although the DNA profile does suggest we share genes through a common ancestor in Africa, 600, 000 years ago.
The earliest hominids
Our divergence from the ape family probably began to happen rather less than 8 million years ago when we had a common ancestor who ate fruit and nuts and occasional meat for extra protein and who lived mostly in trees. The famous "Lucy", found in Ethiopia by Donald Johanson's team and named after a Beatles song Lucy in the Sky with Diamonds is much nearer to us than this common ancestor with apes. Lucy lived a bit over 3 million years BP (BP = before the present). She was tiny, one metre or so tall, but walked upright. Hominids like Lucy left their footprints in volcanic ashy mud at Laetoli in Tanzania. There were two or three in the party, their tracks showing clearly that they were walking on two legs. In the last decade or so, several of Lucy's forebears have turned up in the same general area and have extended the hominid record back towards the common ancestor.
Lucy's specific name is Australopithecus afarensis, since she was found in the Afar triangle, a dry arid region of Ethiopia now, but wetter then with gallery forests along the rivers. Lucy had a wide pelvis, a cone-shaped ribcage and relatively long arms. Her skull was smallish and her jaw V-shaped. An ape jaw is a 3-sided rectangle and a human jaw is an arcade; Lucy's was in between in shape.
"Lucy" from Johanson & Edey (1990)
Lucy's great age and the fact that she walked very much like us was considered remarkable. She was indeed a "missing" link. But we now have some older missing links. One of them, dated to approximately 4 million BP has a knee joint which suggests, from the shape of the tibia, that she/he walked upright. An even older fossil, at 4.4 million years is Ardipithecus ramidus (ramidus = root) which has teeth that show the beginnings of human-like features. Teeth are very telling in the unravelling of the characteristics of fossil hominids. Analyses of rates of eruption of teeth suggest that the human pattern was a very late development, so that most of the hominids mentioned here had ape-like tooth development patterns. Evidence for erect walking shows the opposite trend. Some body bones of A. ramidus have been found and are under study and it may turn out that this hominid also walked upright. These fossils show us species in the process of change, as do the ones that came after them, presaging ourselves.
In the 1920's, Raymond Dart, an Australian, found the skull of a child of about six, known as the Taung Child. He named the genus Australopithecus. The anthropological world regarded the hominid status Dart gave the fossil with scepticism because it had some ape-like characteristics, but there were also many human-like traits in the teeth and skull, which contained the fossilised brain.
"Taung Child" from Jurmain et al (1998)
When other finds turned up that were similar, Dart was eventually vindicated. One of the finds, named, curiously, Mrs Ples, was of the gracile, light-boned type as was the Taung Child. A robust strain and a gracile strain can be seen running right through the hominid record, a bit like plough-horses and racehorses. This genus extended well to the north, to Olduvai, of Leakey fame. Louis Leakey found a robust australopithecine with a keel on its head like Athena's helmet. He promptly named him "Dear Boy", but he was also known as Nutcracker Man because of his huge molars. Dear Boy and others like him lived about two million years ago and roamed from the Red Sea to the Cape Province.
"Dear Boy" from Jurmain et al (1998)
The genus Homo
In 1964 Louis and Mary Leakey assigned contemporary fossils to the genus Homo after having found four individuals along with their primitive stone tools. This is a marker point, the beginning of the genus Homo, the first intentional tool makers, not just opportunistic tool users like chimps cracking nuts with a stone. Because they made tools, these hominids were called Homo habilis, meaning handyman. Louis Leakey was looking for human-like ancestors which were very old. He wanted to live down the Piltdown scandal and find a big-brained skull to show genuine antiquity of human characteristics. Louis was delighted when his son, Richard, found a large brained fossil related to habilis which came to bear the title of ER-1470 (ER stands for East Rudolf, the former name for Lake Turkana). A notable school of thought regards ER-1470 as type specimen of a separate species, Homo rudolfensis. The first dating result suggested that this fossil was about three million years old, but a firm revised date is 1.9 million BP. The verification process for this famous fossil involved finding bush pig fossils of known date alongside the hominids. As the pigs were younger than the proposed three million year date, strata samples were rechecked. The new revised date made better sense because it was a good fit with the known evolutionary time scale. It is interesting to follow the activities of the anthropological teams and where they worked. All sorts incidents are related about "digs" in popular books, packed with human interest and excitement.
The line leading to Homo sapiens
To continue the "succession", 1.5 million years ago was the heyday of Homo erectus and/or Homo ergaster. Homo ergaster was a light-boned (gracile) hominid, while Homo erectus tended to be hyper-robust. The gracile "Turkana boy", found at Lake Turkana, Kenya, and dated to 1.6 million BP was light-boned and long-legged. He was about eleven, already tall and slim like a Zulu or Masai boy. He and his kind probably led to modern humans. Homo erectus was a great coloniser who penetrated as far as China and Indonesia at a time of low sea level when you could walk most of the way from Singapore to Timor with only short sea crossings intervening. This was never so for Australia, which cannot be seen from Timor.
There are anthropologists who think that Homo erectus evolved in situ into Chinese, Indonesians and Australians, though this view is losing ground to the camp who are convinced that the cradle of humankind was Africa. The geneticists say so too. This means that there was a later wave of colonists who were anatomically human and who penetrated as far as Australia. The date for entry into Australia is currently considered to be about 60,000 BP. Colonists from Indonesia would have had to use seaworthy boats to cross a deep sea trench that would not have been dry land at any past climatic period.
Australia is extremely important in the fossil record because thousands of skeletal parts of the original inhabitants have been found and they are all anatomically modern. The inference is that Homo erectus was halted in Indonesia by the sea, although there is some tentative evidence that 800, 000 years ago he/she got to Flores, an Indonesian island which is separated by a sea trench from the main archipelago, so that getting there could have entailed boats, rafts, logs or riding on elephants! The Australian evidence ties in with the fact that the human race is genetically homogenous. Things like skin colour and hair type are superficial. When it comes to blood analyses, we are all one family.
Some of these hominid types coexisted. For instance, a robust australopithecine, Australopithecus or Paranthropus boisei, coexisted with Homo habilis, and subsequently Homo ergaster, over a period of a million years in both East and South Africa. In the earlier part of the twentieth century, anthropologists did not think there could be more than one human line stretching back into the past. Now we know that it has been much more complex, with the blending of human-like into ape-like physique. And all the time the brain was growing larger and more expensive to maintain. A high protein diet was essential for feeding this exotic organ with blood containing nutrients. Walking upright occurred before this portentous change in the brain and was therefore not motivated by freeing the hands to use tools. That "idea" came later, when "handyman" had evolved a brain with a volume of 600-800 cc.
There is a good deal to be said for Homo ergaster leading to Homo sapiens. A type of DNA called mitochondrial DNA is useful in working out lineages. It is passed down the female line. Mitochondrial Eve certainly had a navel! This puts the Garden of Eden in Africa, about 150-200, 000 years ago. At that time, Neandertals were living rough in Europe. There were glaciers reaching down into France and Italy, which made sheltering in caves essential, probably with fires to warm them and meat kept outside in the deep freeze. Round about 100 thousand years ago Neandertals moved into the warmer Levantine area, where they met Homo sapiens already installed, having come up from Africa. Both humans and Neandertals belong to the genus Homo, but it is possible that they are different species. The question of whether Neandertals and humans interbred is unresolved. Recently, a child's skeleton with hybrid features, aged about twenty-five thousand years BP and found in Portugal, suggests two things; that Neandertals survived for longer than was previously thought in the warmer parts of Europe, and that survivors may have interbred with H. sapiens, although this does not mean that we carry Neandertal genes, since according the the genetic evidence any putative hybrid line has petered out. Neandertals had a brain the size of ours and in some cases larger, but it was differently organised, probably less inventive though perhaps more intuitive.
Another lineage problem is that Neandertals had a pre-Neandertal stage in Europe, known as Homo heidelbergensis. Some researchers insist that Homo heidelbergensis was quite widespread, present from Africa to China. Controversies between anthropologists also revolve around whether there was a world-wide Neandertal phase. However, these differences do not amount to altering Darwin's theory of evolution. That is rock solid.
And so our human characteristics have been gradually amassing over a period of at least four million years, with many false starts and side branches, consolidating into their present form about 150,000 years ago. Around 30,000 BP we had the dubious distinction of becoming the only hominid. The ultimate success of Homo sapiens has to be measured against the length of time our species lasts. But how did we begin? How did we gain ascendancy? Although we can probably be exonerated from genocide of the Neandertals, we have progressively destroyed a great many animal species and world environments. However, our species has pragmatic ingenuity in abundance which means we are often able to rectify our mistakes and avert complete catastrophe. We also invented a very flexible system of communication - language. Language is closely linked to awareness of self, a development which almost certainly took place even before we became human.
A chimpanzee's adventure in self awareness captured on film, shows its dawning realisation that the chimp in the mirror is not another, perhaps hostile, chimp, but miraculously, itself. It is graphic proof of the origin of our consciousness. The chimps are first favourites at Taronga Zoo. They draw the biggest crowd. People marvel at their agility, but an over-reaction is to laugh uproariously at their antics. The resemblance is uncanny enough to cause disquiet. Laughter may be a cover for embarrassment. On the other hand, the apes, now in danger of extinction, are a wonderful link with our past; a way to help us feel part of the animal world, an absolute imperative if we are to survive.
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