Fewer than 500,000 years ago Earth was populated by a variety of different human species. All of which, evolved from the Homo erectus common ancestor. Each species belonged to a kingdom and all kingdoms were ruled by the King of the First Men from the Iron Throne. Advances in genetic sequencing technology have fuelled a number of breakthroughs in the study of these archaic humans, arguably raising more questions than they answer.
“We don’t necessarily need ancient bones to find ancient DNA.”
Josh Akey (Associate Professor of Genome Sciences at Washington University)
Fossils are no longer our only means of probing extinct life. We now know the long-term survival of DNA is a general phenomenon. Worries of contamination or degradation of DNA over such significant periods of time have been mitigated through hard work in the lab as well as technological leaps in the genetic sequencing of time-ravaged DNA. This has helped tell us the story of the ancient humans, in particular a couple of the more extensively studied species – Neanderthals and the Denisovans.
Where did they come from?
All archaic human (sub-)species derive from a single common ancestor species – Homo erectus (hehe). Homo erectus were around more than nine times as long as our own species. They are the oldest known species to have the more human-like body proportions, such as elongated legs and shortened arms in comparison to the torso. These features are considered to represent adaptation to a life lived on the ground. No tree climbing, walking on two legs and running long distances. They also possess expanded brain cavities which potentially drove the invention of stone tools such as the hand axe.
A recent discovery has actually pushed back this timeline by 500,000 years. Scientists uncovered a lower jawbone fossil in the Afar region of Ethiopia, which they estimate to be around 2.8 million years old. However, scientists are uncertain whether this stands alone as a predecessor to Homo erectus or not. Nevertheless it’s an important transitory fossil and could represent the earliest form of human.
What are the Neanderthals?
Neanderthals are an ancient human species, very closely related to our own. They existed around 200,000 to 30,000 years ago and lived predominantly in Europe and Asia.
So what is it that makes Neanderthals so different from us? This question is best answered at a molecular level.
Molecular studies work on the principle that: if early human populations bred with Neanderthals then their descendants should have short segments of archaic-human DNA. These can be studied by identifying specific regions of the human genome which have been acquired by interbreeding (in addition to extracting ancient DNA from old bones).
In doing so, it was found that some Europeans and Asians carry Neanderthal genes involved in skin and hair production[3, 4], which perhaps helped them adapt to colder environments through thicker skin and hair and fewer pores. But there aren’t any particularly large segments of Neanderthal DNA in our genomes which implies their DNA could be harmful. A fitness cost to interbreeding.
The epigenome describes chemical changes to DNA and DNA-associated proteins. These changes are heritable. There is approximately a 1% epigenetic difference between modern humans and Neanderthals/Denisovans and is thought that this 1% comprises uniquely human traits. Remember, we share more than 95% of our genes with mice, so 1% is major!
These studies have enabled scientists to compare which genes are “switched on” in us but turned off in Neanderthals. Most significantly, modern humans evolved differences in genes to do with cognitive abilities. Evolutionarily, this could have meant better communication, social interact or cognition and therefore a fitness advantage in terms of natural selection. But in order to really prove these effects, we would need to express hominid DNA in human cells and assess their behaviour.
Who were the Denisovans?
The Denisovans are another better studied archaic human. They are thought to be a more primitive Homo species. In fact, when a Denisovan tooth fossil was discovered, it was initially mistaken for a cave bear tooth due to its large size. It wasn’t until the DNA inside the tooth was sequenced that they realised it was actually human. Despite being more closely related to Neanderthals, they too have left their mark on us.
Old World Hanky Panky
“What this begins to suggest is that we’re looking at a Lord of the Rings-type world – that there were many hominid populations.”
Mark Thomas (evolutionary geneticist at University College London – my lecturer!)
It is thought that there were a handful of interspecies love affairs between modern humans and Neanderthals. The results have made us what we are today.
We share a number of similarities with our sister species but their genetic legacy also highlights how different they were. Neanderthal DNA is found in some of the most variable regions of our genome, making it a prime target for the forces of natural selection. Beneficial changes will be favoured, while deleterious ones will be selected against. For instance, we find almost no Neanderthal DNA on the X chromosome (the female sex chromosome), which suggests it made the carrier less fertile. We often observe this between related yet distinct species, underlining “that modern humans and Neanderthals are indeed different species,” according to Fred Spoor of the Max Planck Institute. Furthermore, we find that Neanderthal DNA is irregularly spaced throughout the modern human genome, rather than being fully mixed which implies that interbreeding occurred very rarely. But it is thought that this interbreeding helped us survive harsh climate changes.
“But these relatively few matings obviously were an important event in the history of non-Africans.”
David Reich of Harvard Medical School
But what of the Denisovans?
Denisovans split away from Neanderthals as their common ancestor left Africa. They migrated towards Asia, while Neanderthals moved across Eurasia. The Asian overlap allowed for interbreeding about 67,000 to 46,000 years ago. But why there is seemingly no trace of them in Chinese or other Asians from Siberia to Melanesia is puzzling. There is also very little fossil evidence to date so we have little idea of what they look like. This might be because their population was never very large. This is suggested by the relatively little genetic diversity between Denisovans and it is thought they suffered a dramatic decline around 125,000 years ago.
“We don’t have the faintest idea.”
Chris Stringer (Paleoanthropologist at the Natural History Museum)
Nevertheless, their legacy lives on. For example, some Australasian populations such as Papua New Guineans share a small portion of their DNA with Denisovans. Despite their small population size, these guys got around. Genome sequences from ancient bones have shown that the Denisovans also had sexy time with Neanderthals and Asian ancestors as well as another, unidentified hominid population! But who they are, we have no idea. The community is abuzz with speculation of the identity of this unknown population of humans and I’m dying to find out who they are. Maybe and Elven race?
What can we learn from them?
Genetic studies show how different we are what it is that makes us, us. Questions like what defining changes took place after we separated from our common ancestor helps to find what is exclusively human.
Denisovans tell us things about our own lives. Svante Pääbo (evolutionary geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany) came up with the “genetic recipe for being a modern human,” by comparing the more primitive, Denisovan genome with modern humans. Some of these changes were also found to affect brain function and nervous system development. Among them are genes involved in language and speech.
So what happened to them all?
“Neanderthals met a violent end at our hands and in some cases we ate them.”
Fernando Rozzi (of Paris’s Centre National de la Récherche Scientifique)
Modern humans ate them and made necklaces from their teeth. This suggestion has come from the discovery of a jawbone apparently butchered by modern humans. The butchery resembled patterns seen on deer cut up by humans in the early stone age.
Of course, many disagree that violence served as the means for their extinction. After all, they were a very sturdy and hardy species surviving several ice ages in their 270,000 yearlong existence. A more likely explanation is that they failed to compete for resources or with Homo sapiens or adapt to changing climates effectively.
Nevertheless, while we may not be responsible for their extinction , evidence from jawbone fossils does suggest that humans attacked Neanderthals and ate them. Perhaps some genetic factors contributed to their extinction, such as those around brain development; not brain capacity but rather the underlying neurological differences mentioned earlier.
The next ancient human genome to be sequenced looks to be the Denisovan, but it certainly won’t be the last. This will be helped greatly by new sequencing technologies by increasing the genetic yield from fossils. I cannot help but wonder what the world would be like today with these other species still around. The bible would certainly read differently. But there is some evidence suggesting that Neanderthals could have been today’s scientists and top thinkers because of their bigger brains and use of tools before modern humans. Interestingly, we may one day find out. Some scientists have claimed to know enough about Neanderthal genomes to be able to cultivate one. They just need a willing woman’s womb. And the laws to change of course…
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