Back to Africa
We talk about the history of our African Ancestors, how they all interbred & the diversity present in the African continent today.
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Transcript: Today we are going to go back far into the past. Back to Africa, when our earliest ancestors stood up on two feet and walked into the savannah. I am going to concentrate on the African continent; what happened there and what we are learning now.
We have known that humans shared a common ancestor with the other apes since the time Darwin told us that we did. Over the years, we have managed to figure out the timeline of when that happened. And the latest consensus is that we separated from chimpanzees & bonobos (our closest relatives) around 6 – 8 million years ago.
In 2001, researchers found a skull from the Djurab Desert of Chad. It was given the name Sahelanthropus tchadensis. Its name means Sahel man of Chad but it came to be known as Toumai because of one of the Saharan languages spoken in the region. Toumai is 7 million years old and could be an ancestor to both humans and chimpanzees. Toumai is important because he may have walked on two legs and this means that he is a human ancestor. This was interpreted from the skull because in humans the top of our spine meets our skull at its base – a sign of bipedalism – while in our 4 legged cousins it meets it at the back of the skull. Toumai’s skull seems to have this opening at the base. Just like humans. But there is a caveat. We only have the skull, so we can’t be sure. Plus a leg bone was also found close to Toumai, which seems to have completely disappeared. So, there is a lot of confusion regarding Toumai. However, the opening at the base of the skull does indicate a difference from the usual ape morphology.
From the Sahel man we move forward one and a half million years and find two more fossil species from the genus Ardipithecus. One lived 5.6 million years ago and the other around 4.4 million years. Again, due to lack of a complete skeleton there is controversy as to whether these two can be classified as human ancestors. They do share many characteristics with chimpanzees. But some studies argue that they should be classified as human ancestors because one of the skeletons – known as Ardi – and found in 2009 indicates that this species was quadrupedal when up in the trees but bipedal when moving on the ground. The teeth are also more generalised than modern apes AND their hand bones and brain organisation also indicates similarities to humans rather than other apes.
Post Ardipithecus, we have the Australopithecines, the most famous of whom is Lucy. Researchers uncovered her fossil skeleton in the Afar region in Ethiopia in 1974. Evidence suggests that the Australopithecines evolved around 4.2 million years ago and went extinct about 1.9 million years ago. So, they were around at approximately the same time as Ardipithecus. And controversy over classification aside, this should not surprise us. As we will find out, several species of hominins have existed together throughout history. In Africa and out of it. There were a number of Australopithecus species roaming around Africa at the same time or overlapping each other.
Lucy is about 3.9 million years old. FUN FACT: Her name is Lucy because the Beatles song Lucy in the Sky with Diamonds was playing all day long during the dig.
Australopithecus species are important in the context of human evolution because most scientists believe that our genus Homo was derived from them. A. africanus for example could have given rise to A. sediba, which in turn may have evolved into Homo habilis. Australopithecines are apparently the first species that we know of that had genes associated with increasing the length and ability of neurons in the brain. Our thinking brain may very well have started with Lucy, although her species’ brain was only 35% the size of ours. And of course Lucy and her relatives were bipeds – they walked on two feet. A study suggested in 2010, that they used stone stools to butcher meat. We can now see the start of our lifestyle evolving, although they probably relied on fruit and vegetable more than meat.
Fossils of Lucy’s species: A. afarensis show significant changes in the hands approximately 3 million years ago; the fingers shortened relative to the thumb and the hand joints also changed.
I said before that one of the Australopithecines – perhaps A.sediba evolved into the Homo genus – perhaps into Homo habilis. Habilis were around during the early Stone Age about 2.1 to 1.5 million years ago and they also used stone tools. Furthermore, they existed with another genus called Paranthropus (similar to the Australopithecines) and the next one in our genealogy Homo erectus.
Homo sapiens think that we are extremely successful but we have nothing on the Homo erectus. These guys were around for 2 million years and they left Africa before us. Their fossils have been found as far south as Indonesia – with the youngest fossils described in December last 2019 from Java, which were just over 100,000 years old. Therefore, they also overlapped with modern humans. Homo erectus are considered our direct ancestors as well as many of our relatives such as Homo heidelbergensis, who spread all over Eastern and Southern Africa and in Europe and also of H. antecessor whose fossils have been found in Europe and who were around 1.2 to 0.8 million years ago.
Remember these are not necessarily direct lineages. There is enough fossil gap that anthropologists cannot be sure whether one species was an ancestor of another or not. And many were around at the same time. For example in December 2019, researchers found a nearly complete skeleton of A. anamensis. This species was initially thought to be the ancestor of Lucy’s species but this skeleton seems to be 3.8 million years old and the timeline suggests that Lucy’s kind and A.anamensis overlapped for at least 100,000 years.
One of the fossil finds of Homo erectus is known as Nariokotome Boy or Turkana Boy (because he was found by Lake Turkana in Kenya). His is a nearly complete skeleton and it shows that he was about 11 to 12 years old and lived around 1.5 to 1.6 million years ago. He seems to be completely bipedal as opposed to the earlier ones that tended to walk on the ground but also live on trees. In him we also see a bigger brain size. This skeleton and the large collection of stone tools found in and out of Africa point to Homo erectus being efficient hunters and probably had a complex social structure.
Remember we are still in Africa and now from all of these ancestors and cousins we come to our species Homo sapiens or WISE Man. We came into the picture around 200,000 years ago in Africa, although new fossil finds from Morocco suggest it could have been as early as 300,000 years ago.
Today, we are the only species left out of all of our relatives but we lived with many of our other relatives at various times in history, such as Homo naledi – skeletons of which were found near Johannesburg, South Africa in 2017. We know this because Homo naledi lived between 350 to 250,000 years ago, which coincides with the timeline of modern humans.
We are not special. We also interbred with many of our cousins. Modern humans mixed with Neanderthals – everyone knows that. What most people don’t know is that we also mixed with other species in Africa.
When we think of the human story, we tend to concentrate on our movement once we left the continent and think of African populations to be lacking in diversity. This is completely at odds with modern genetics, which has shown that there is more genetic diversity in Africa then there is out of it because our species evolved in Africa.
But we still don’t know much about our African ancestry in terms of genetics. The problem is that although more than 1,000 ancient human genomes have been analyzed, fewer than 80 have been from Africa, even fewer older than 10,000 years. Why? Because it’s hot and humid in Africa so DNA doesn’t get preserved. Plus it’s a huge continent with many political and security issues so it is hard to get to far off places. Earlier, there were also problems with methodology, which now seem to be overcome. Since, 2015 scientists have shown ancient DNA can be preserved in the petrous bone of the inner ear.
In 2018, researchers used this bone to recover the oldest African genomes yet, from seven 15,000-year-old skeletons excavated from a cave in Morocco. They were found to be most closely related to populations from the Near East, with a third of their ancestry from sub-Saharan Africa.
Then in 2019, whole genome sequencing of 15 African populations covering all major continental linguistic groups, ecosystems, and lifestyles revealed archaic gene flow of 4 – 6% from an extinct ghost population into 4 sub-Saharan populations. This means that ancestors of modern sub-Saharan Africans interbred with a population of which we know nothing except that it was with an early divergent and now extinct ghost human lineage.
Earlier this month, in 2020, researchers from the University of California in LA analysed 405 genomes from 4 West African populations and found genes of another unknown ancient African population in people alive today. Up to a 5th of the modern populations DNA seems to come from these unknown ancestors, who probably lived around 500,000 years ago. It seems this ghost population split from the ancestors of Neanderthals and modern humans between 360,000 and 1m years ago. A group of perhaps 20,000 individuals then bred with the ancestors of modern West Africans at some point in the past 124,000 years.
In January this year another study came out, which analysed the genome of 4 children from West Africa – specifically Cameroon. Although, sub-Saharan Africans are the most genetically diverse in the world, many share similar ancestry due to the Bantu expansion. This migration happened about 3 to 5000 years ago from West Africa and spread agriculture and language across the continent. Compared with the DNA of modern Africans, the genomes of these four children related more closely to those of the hunter-gatherer groups in west central Africa known as Pygmies rather contemporary Cameroonians or other Bantu-speaking populations. It seems that these children descended from a group of Homo sapiens that branched off more than 200,000 years ago. Indigenous southern African populations are known collectively as Khoesan peoples and previous studies had revealed that these groups descended from the oldest distinct Homo sapiens lineage. However, these four children now show that their lineage was even older.
As more petrous bones are analysed we will perhaps find out more about our ancestors interbreeding with other ancient African species. Oh and as we come to modern history, we now find that even Africans have higher than expected Neanderthal DNA given to them via back migrations from Europe in the last 20,000 years ago.
See how complicated it gets? Not only did a majority of our evolution take place in Africa, the diversification of many modern human lineages also happened there. Africa is extremely important to our story. We are sure to find out more as we improve our technology and methodologies. But one thing is very clear, there is no garden of Eden, even in Africa. Where our species first appeared has never been established. Initially, anthropology and genetics indicated East and South Africa. It was a nice story – of a band of brave and smart Homo sapiens that existed in some lush little corner of Africa and from there went on to colonize the whole planet. That story doesn’t seem to gel. Slowly we have come to realise there was no Garden of Eden. Archaeologists, anthropologists, anatomists and geneticists now all seem to agree that based on fossils remains, burials, art spread all over the continent, modern humans rose up in a number of different places in Africa. Our immediate ancestor, lived 500,000 years ago and evolved into separate populations, some went extinct and some spread. They met each other and mixed together and then separated again and again, sharing culture, physical features, and genetics with each other. And finally, ended up being Homo sapiens. The reason for this is because unlike other animals not only are we a very mobile species, we interbreed a lot and we keep our social connections. Each group had their own traits, practices and behaviours, which they shared with each other as they intermingled over and over again. We didn’t just occupy one corner of Africa, we were pan Africans. Now we wait to discover other populations back to the 7 million years of our evolution – because they MUST be out there waiting to be discovered.
Music: Moonrise by Chad Crouch – Instrumental from Free Music Archive.
Title image: Evolution Image by Oberholster Venita Pixabay. Featured images: Wikemedia Commons (Public Domain) and 360onhistory.com