Humans and Evolutionary Interbreeding

Ever since the Human Genome Project was completed, scientists have realized how close we actually are to other organisms. It was no surprise that the animals we shared the most DNA with, were closer to us on the evolutionary tree. These of course, are the primates, like Chimpanzees, Gorillas, and Orangutans. Since the completion of the Human Genome Project in 2003 scientists have discovered even more about human genetics and our origins. One of the most surprising aspects of our genome, that has shaped us over the years, is interbreeding. Scientists have only recently found out how much interbreeding has actually distinguished us from other organisms and how important it has been to our survival.
The first case of interbreeding occurred nearly 9 million years ago, shortly after the split between chimpanzees and humans. When the Human Genome Project was done, scientists discovered that there was only a 1.5% difference between the two. Even our structural proteins were nearly identical. In fact, human and chimpanzees have identical blood. Scientists realized that there were too many similarities to account for a split nearly 9 million years ago. Scientists hypothesized that humans and chimps must have interbred after the split. They knew that signs of interbreeding, or hybridization, would show on the X chromosome. By looking at the Human Genome research they found that the newest genes did in fact occur on the X chromosome. Scientists were able to reason that, even though we split with chimpanzees nearly 9 million years ago, we continued to interbreed with them until approximately 5.4 million years ago.
An even more groundbreaking discovery came more recently when researchers began decoding the DNA of Neanderthals. Since Neanderthals had already been out of Africa for nearly 200,000 years, scientists had never thought interbreeding had taken place. However, they found that not only was Neanderthal DNA less than 0.5% different than human DNA, but also that most modern Europeans and Asians have about 3% Neanderthal DNA in them. Neanderthal DNA did not occur in Africans. This meant that when modern humans emerged from Africa they encountered Neanderthals from the east and interbred. This interbreeding has been confirmed several times since, after Neanderthal-Human hybrids have been found. Scientists have used this evidence to explain genetic disorders and diseases that are characteristic to only certain ethnic groups. For example, scientists found that men with Neanderthal DNA were much less likely to contract prostate cancer. This could explain why it is much more prevalent in African men. Scientists found that Neanderthals, and other hominids like Denisovans, had stronger immune systems. They had been out of Africa for nearly 200,000 years already and had undergone significant DNA evolution. Interbreeding with them allowed homo sapiens to be better suited for the new environment.
Perhaps it was these superior traits that attracted homo sapiens to Neanderthals. After studying Neanderthal fossils and DNA it became clear just how much better Neanderthals were than humans. Their brains were one-fifth larger, they were taller, stronger, and most likely smarter. They were even better looking. Their high cheekbones, strong features, and facial structure are similar to what we find in the supermodels of today. Scientists found that Neanderthals were even the first blondes. It is no wonder homo sapiens desired to interbreed with them. It's a good thing too, because their genes made us stronger as a species.
One may wonder why such a strong and intelligent species like homo neanderthalensis died out, and a weak, new species like homo sapiens thrived. The answer of course lies in the genome of the Neanderthal. Anthropologists extracted a gene that modified the chemical make-up of certain parts of the brain that affected personality and thought. They found that Neanderthals were most likely too cautious and conservative. They resisted change and therefore could not adapt well over time. This eventually led to their downfall. This resistance to change was perhaps caused by a shorter development period. By looking at fossils of their teeth, researchers found that Neanderthals matured faster and had shorter life cycles. This could have hindered development of portions of the brain. They eventually lost out in the competition with homo sapiens.
Another, much smaller group that interbred with modern humans were the Denisovans. They were a much more recent branch on the evolutionary tree and were located in the far east and Siberia. Denisovan DNA only appeared in modern Pacific Islanders, like those from New Guinea, the Philippines, and Australian Aborigines. It was present by over 5% in some peoples. This meant that interbreeding was more common and more recent than with Neanderthals. The vast range of those with the genes also showed the large movements of the Denisovans.
Scientists are just recently starting to shed light on the interbreeding that occurred thousands of years ago. As they find more and do more research they continue to open the door to a possibly new field of study. By looking for traces of interbreeding they can learn more about our origins, find health trends, and learn about ancient species. By looking at our past of interbreeding scientists can learn more about who we are as homo sapiens- as humans.