Yes, I am back on anthropology rather than medicine.
Remember this theory? Man evolved in Africa from a chimp like ancestor which lived in trees, about 6 million years ago (mya). For some reason (climate change perhaps?) he came down from the trees, went into the savannah where he immediately learned to walk upright, and proceeded to develop hunting skills. These required a bigger brain, and so his skull enlarged too. The rest is history. All palaeo-anthropologists had to do was find the relevant bones in the right places and construct an evolutionary tree.
BUT THEY HAVE FAILED TO DO IT.
The three things that make a modern human are: – a big brain, and hence a big skull; a smaller mouth and teeth, indicating a softer diet; and longer leg bones with pelvis and leg bones adapted, not only to walking, but running long distances. So, to fill in the gaps scientists would need to find a neat line of skeletons in various parts of Africa which showed the development of the brain case, evidenced by the skull and face shape, gradual loss of limb adaptations to living in trees such as prehensile big toe, and changes in the knees to allow modern walking and running.
At first all went well, with the discovery of Lucy, Australopithecus afarensis, in 1974. She lived at the right time, 3.2 mya, and place, in the Afar region of Ethiopia, to be considered the ancestor of us all. She was partly bipedal, so possibly on the way to becoming completely so.
If you go to any museum in the world that gives a history of the evolution of human beings, this is what it will say.
But discoveries of three separate hominid bones in the late 20th century and 21st century have not been kind to this theory, and now many scientists are saying that they are incompatible with the savannah hypothesis, and so it must be discarded.
These three discoveries are;
7 mya – an ape skull with some human like features found in Chad; therefore named Sahelanthropus tchadesis. It was found in 2002;
6 mya – a species the size of a chimp with human like teeth, found in the Tugen Hills, central Kenya, named Orrorin Tugenensis which lived 6 mya (nickname: Millenium Man). This was found in 2001 and had an upper femur, showing evidence of bone build up typical of a biped. So these individuals climbed trees but also probably walked upright with two legs on the ground.
5.5 mya – Ardipithecus, found in 1997, which was an almost complete skeleton of early ‘proto-human’ sharing traits with chimps and gorillas, which was also partly bipedal.
All these finds were of skeletons which had some traits that were more human than Lucy, yet were much older (Lucy dates from 3.2 mya). This was very confusing, and cast doubt on the molecular clock dating mechanism which had given the split between apes and hominids at about 6 mya. None of these three earliest skeletons could have shown these human changes by then if the split was so late. So the dating mechanism had to be tweaked. Lo and behold, some of the assumptions in that calculation were found to be suspect, and scientists decided that the split could have been any time between 13 mya and 7 mya. We may have begun our journey much earlier than the 6 million years ago that has been accepted for many years
It was clear years ago that the savannah theory had serious flaws – quite apart from the fact that the savannah came and went and wasn’t there at the time humans were supposed to be evolving. But now we have archeological evidence that there is no linear development of skulls and limb bones which show that hominids came down from the trees, started hunting, walking on two legs and developing skills which needed a large brain. Other recent finds have confirmed that individual hominid remains could have a mixture of primitive and advanced features previously thought to belong to different species, so that there was no tidy sequence of human skulls, teeth and leg bones indicating the transition of humanity in this way. These other species were
Kenyaanthropus platyops found in 2000, contemporary with Lucy, but seemingly closer to modern humans,
Homo Naledi, 2.36 mya, found in 2015, which has a tiny brain but otherwise morphologically human,
And two Australopithecus skeletons dated at 2.2 mya .”In reality, there may have been a variety of evolutionary branches, each developing unique suites of advanced human-like features and retaining a distinct array of primitive ape-like ones.” said Lee Berger at the University of Witwatersrand in South Africa, who led the analysis of two of the most recent finds. So we cannot be certain (if we ever could) that any of these skeletons were our direct human ancestors and so we cannot use anatomical differences to trace our human journey in Africa or anywhere else. Many of the recent discoveries may not have been our ancestors and we don’t know which ones are.
I am delighted that the Savannah hypothesis is now on its deathbed. Scientists, apparently, are still arguing over this, trying to reconcile the findings with a sensible theory. But I don’t know that they ever will if they confine themselves to the study of archeological remains. Yes, paleo-anthropology has inevitably meant the study of bones. Until recently that was all scientists had. But the study of bones would never tell us how we functioned so differently from apes. To me, as a doctor trained to look at how people functioned, it is physiology, the functions of the body which tell the real tale. When and how did we get the ability to speak? Was this ability the cause or the result of our increasing brain size? Why do we get so obese? Yes, we store fat under, and linked to, the skin unlike any other apes, but why? Why and when did we lose our hair? How did our ancestors get enough protein to enable brain development? Does the theory of why we sweat, rather than pant, when we are in danger of over heating, really stand up? (It involves early man having to go out and hunt in the mid day sun amongst other things).
These are all questions not generally considered by anthropologists (and certainly not answered by them), but are just as important I think to understand how Homo sapiens evolved. To my mind, what makes us human has never been the fact that we walk upright. It is the fact that we can communicate with each other with a sophisticated tool – language, that separates us off from apes.
I think t is clear that it will have to be DNA that tells the true story, which may finally give some answers to the physiological questions above.
So if we consider that the split between the ancestor of chimps and the ancestor of man could have been as long as 15 mya ago, we would have to look at the conditions on earth at that time. We know that 15 mya, called the Mid-Miocene Climate Optimum, was when the climate was as much as 4 to 5 degrees Celsius (or 7 to 9 degrees Fahrenheit) above today’s average temperatures. (This also seems to be where the earth is heading after humans became the most successful species on earth in terms of being able to alter the environment). There were many species of large ape around at the time, and we need to find out why the two branches split. What changed? Even if you don’t go along with my mini-obsession with aquatic apes, the theory that some apes had spent time in the water, resulting in changes to their physiology in order to adapt to a more aquatic way of life, we will have to find another theory that explains the fact that hominids diversified into a whole new series of anatomical changes which did not relate in any way to the story of man the hunter in the savannah.
After that split in DNA, which ultimately led to a talking, intelligent ape, that group of human ancestors could have gone into a number of new habitats. They could have been living at riversides, with nearby forests, they could have been on more open plains, they could have lived in caves, or by the seaside. We can’t be certain. But whether they habitually walked on two legs or not would have been irrelevant. It would depend on where they were living, and some would still have adaptations for living in forests. Some of the later finds have smaller brains than the early ones we have assumed were human. Are they human? In many ways they are. Does that mean that language growth and skull size aren’t related either? It could be. The original change of starting to speak would not necessarily have led to a bigger brain – the language area is only one small part of the brain, and the bigger brain could have come from what we then did with our brains as we developed further skills. Or it may reflect better nutrition.
It is a very exciting time for those interested in human evolution. Of course many scientists will carry on supporting the savannah theory despite the new evidence. I hope new breakthroughs will come thick and fast, in DNA and paleo-anthropology, to make further progress in this fast moving field.
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Elen Samuel, doctor and writer 