From about 2 million years ago until about 10,000 years ago, the world was populated by at least 6 different human species. They evolved from a common ancestor in East Africa, a hominid called Australopithecus (Southern Ape). Over thousands of years these primitive humans migrated to regions in North Africa, Europe and Asia. It is likely that environmental changes initiated the exodus, and as time passed new opportunities opened up in other lands. The diverse environments caused humans to evolve different survival traits, eventually branching out into several species.
For many years vast distances separated each species, which allowed them to survive independently. For instance: Homo neanderthalensis (Neanderthals) occupied regions in Europe and the Middle East, and Homo denisova (Denisovans) settled in Asia. Homo erectus (Upright Man), the human species with the most longevity (around 2 million years), populated eastern Asia. And a few species, including Homo sapiens (Wise Man), continued to evolve in East Africa. How closely related to Homo sapiens these other humans were is difficult to assess. How were they genetically different? What were their mental capabilities? And how complex were their social structures?
Nevertheless, starting at about 70,000 years ago Homo sapiens began moving north from Africa; they spread into the Arabian Peninsula and Eurasia. This led to direct competition with other humans. It is difficult for anthropologist to piece together what actually happened in the ensuing millenniums. But the Neanderthals became extinct about 30,000 years ago, and all other humans also disappeared (except for the sapiens). The extinction of Homo floresiensis in Indonesia (13,000 years ago) ended the last of the other human species.
Two Possible Theories
1) The Interbreeding Theory: When Homo sapiens encountered other humans they coexisted peacefully. The species were genetically close enough that they could have interbred. The result being that today’s human population is not pure Homo sapiens, but rather a genetic mix of humans that lived 70,000 to 30,000 years ago.
2) The Replacement Theory: In this scenario, the genetic difference between species was too great to allow for interbreeding. Or possibly the sapiens’ way of life was drastically different from the others, and they had no interest in mingling with them. Or more likely, there would have been an intense competition for resources. Homo sapiens were the winners in a battle for survival. One could entertain a number of possible ways in which the battle could have been fought and won.
Recent evidence has shed light on the competing theories. In 2010 Neanderthal DNA was extracted from fossil remains. Enough genetic material was still intact to map out the Neanderthal genome. A comparison with modern human DNA revealed that 1-4 % of the DNA of people from the Middle East and Europe is Neanderthal DNA.
Several months later a similar analysis was performed from another primitive human. A sample from the Denisova cave in Siberia showed that about 6% of its DNA was found in modern Melanesians and Aboriginal Australians. The Neanderthal and Denisovan findings prove that some interbreeding did occur, but the amount of DNA in modern genomes is still small. This suggests that interbreeding was not the whole story.
The species may have been at a transition phase, in which they were not completely separate species, but merging of populations was rare. The replacement theory still carries a lot of weight in explaining why about 95% of our DNA is pure Homo sapiens. The conclusion being that sapiens essentially drove the other species to extinction. But what was the crucial difference that resulted in one species dominating the landscape?
The Story of Homo Sapiens
When scientists are uncovering evidence from per-historic times there are bound to be gaps in knowledge. Therefore, a fair amount of speculation comes into play. The rise of Homo sapiens from an insignificant animal to one that claimed the globe is remarkable. Especially when you consider that other humans, as far as we know, started out with the same opportunities.
What unique attributes enabled Homo sapiens to become the only human species? Although we are so accustomed to a world with only one human species, it is the rarest of exceptions in nature. In the animal kingdom there are many species of cats, birds, turtles, and whales. Only in modern humans do we find a single unique species.
In the book, A Brief History of Humankind, historian Yuval Noah Harari identifies one critical sapiens trait that allowed our human ancestors to conquer the world. He calls it The Cognitive Revolution. According to Harari, prehistoric sapiens had evolved a rare ability to cooperate in large numbers, and to do so flexibly.
It was the development of complex language and social structures that set them apart from other humans. They could communicate everyday practical information, such as where and how to hunt and gather berries. In addition, myths, gods, legends and religions emerged at this time. Whether fact or fiction, storytelling allowed large groups to unite and work for a common cause. Stories also made it possible to pass on knowledge and wisdom to the next generation.
Other animals also work together in groups, but their behaviors are inflexible. In order for significant changes to come about, genetic changes have to occur through the process of evolution. This takes a long time, and that is why animal behavior remains consistent from one generation to the next. But this is not the case for modern humans. Our history reveals an unprecedented pace of change with each generation. For the first time in the history of life sapiens were able to adapt using cognitive abilities. Today, humans are the only species that can survive in all land environments (from rain forests to desert to the arctic). This is mainly due to our adaptability.
Taking Over the World
When the first wave of Homo sapiens arrived in Neanderthal territory, about 100,000 years ago, the Neanderthals forced the sapiens to retreat. Evidence shows that the Neanderthals had large brains, muscular bodies, could withstand cold temperatures and lived in groups. But it is likely that they could not organize in large groups, or share knowledge in the same way sapiens did. 70,000 years ago a second wave of sapiens left Africa and overran the Neanderthals. This time there was no turning back; Homo sapiens gradually settled much of the globe, and all other human species disappeared.
As Homo sapiens discovered new lands they found an abundance of large animals. This may have been fortunate for the humans, but not for the animals. The archeological records show that roughly 1/2 of the large land mammals became extinct during this period. Climatic or environmental changes may have contributed to the extinctions, however, the human invasion is hard to ignore. In every corner of the world, from large continents to remote islands, extinctions followed humans arriving for the first time.
Large prehistoric animals, such as ground sloths, saber tooth cats and mammoths could have been victims of the sapiens success. This was the first wave of extinction caused by human activity. But they could not have known the full impact of their actions, nor could they have imagined the evolution of human civilizations that would follow. Today, our unique cognitive ability separates us from all other animals. It was developed thousands of years ago in an epic battle for world supremacy.
References: Yuval Noah Harari, Sapiens (Canada: Signal Books, an imprint of McClelland & Stewart, 2014)
The Nature of Things: The Great Human Odyssey, (2015).