Our Neanderthal Ancestry

TUESDAY, 21 MARCH 2017

MODERN HUMANS have managed to reach every corner of the globe and with current population sizes rising rapidly, we could stake a claim to being the most ‘successful’ hominid species yet. However, how much has our recent ancestry impacted our evolution, and have our previous interbreeding events helped, or even hindered, our success as a species?

About 100,000 years ago, groups of anatomically modern humans left Africa and dispersed across the globe to reach areas where they encountered ancient hominins, including Neanderthals and another extinct hominin species, the Denisovans. The presence of Neanderthal DNA in non-African modern humans, and of Denisovan DNA in Oceanic populations tells us that these groups interbred. However, only a small fraction of the modern human genome is made up of these ancient human genes. Their genetic contribution is uneven with concentrations particularly high in some areas and completely absent from others.

"Hybridization was not just some curious side note to human history"

A recent study identified 126 different points in the modern human genome where genes from ancient humans were present at much higher frequencies than would be expected if they had no beneficial effect. One researcher - Joshua Akey of the University of Washington, School of Medicine in Seattle - said of the results, “Our work shows that hybridization was not just some curious side note to human history, but had important consequences and contributed to our ancestors' ability to adapt to different environments as they dispersed throughout the world”. These regions of the genome included sequences known to be involved in controlling skin characteristics and immunity.

This runs against the message provided by a study in PLOS Genetics by Ivan Juric and colleagues at the University of California, Davis. His team concluded that the reason we see such small amounts of Neanderthal DNA in the modern human genome (a small fraction compared to 10% at the time of inbreeding) is due to their weakly deleterious effects on modern humans.Thus, when transferred into modern humans, these ‘less-fit’ genes were subsequently removed through natural selection. For instance, one recent study estimated that the harmful mutations in the Neanderthal genome made the hominids less reproductively fit than modern humans.

Not only did this ancient hook-up reduce the reproductive fitness of our ancestors, but researchers from Spanish, French and Brazilian institutions claim that we may have also picked up common sexually transmitted diseases (STDs) from Neanderthals. The human papillomavirus HPV16 strain is estimated to infect 4% of Americans and has been linked to cervical cancer. The study found that this strain is over 500,000 years old. Sequencing five different subtypes enabled researchers to assemble a timeline that suggested it was transmitted to Homo sapiens from our Neanderthal or Denisovan cousins when ancient hominins moved out of Africa – supported by the relative scarcity of HPV16 in Sub-Saharan African populations. “The history of humans is also the history of the viruses we carry and we inherit. Our work suggests that some aggressive oncogenic viruses were transmitted by sexual contact from archaic to modern humans,” said Ignacio Bravo of the French National Centre for Scientific Research.

Image: M M

What implication does this have for current modern human evolution? Are we still evolving? Innovations in science, agriculture and medicine have enabled us to overcome limits on survival imposed by disease, ageing, nutrition and fertility. By the middle of the 20^th century the human population had already reached more than double what took thousands of years to reach previously, and current estimates predict that by the mid-21^st century, there will be almost 10 billion humans on the planet. This ability to eliminate most causes of death and suffering experienced by our ancestors could demonstrate that perhaps we are no longer constrained by natural selection as a species. Have we taken ‘control’ of our evolution? A study published this year led by Yair Field from Stanford University has studied the DNA stored in a large-scale UK project to look for signs of recent evolution amongst British people. Contrary to expectations, it revealed evidence for recent genome changes over the last 2000 years in three important sets of genes, indicating the occurrence of recent human evolution. One set included lactase genes involved in breaking down and digesting dairy products. With the boom in milk consumption in the last couple of thousand years in Britain, it is not surprising that dairy tolerance has increased and the genes in this pathway have been under positive selection. In Asian populations where milk consumption has historically been much lower, we see greater incidences of lactose-intolerance and less genetic change in the lactase genes than those observed in Britain.

The second set of genes identified play a role in the human immune system. The exact reason for the strong positive selection for these is less clear, but it has been suggested that the increase in immigrants over the last 2000 years in Britain, from the occupation of the Romans and the many groups thereafter, exposed the British population to increasing levels of infectious disease placing strong selection on these immune-response genes.

"The genes controlling blonde hair colouration and blue eyes have been under selection for the last two millennia"

Image: McBeth

Finally, somewhat unexpectedly, they discovered that the genes controlling blonde hair colouration and blue eyes have been under selection for the last two millennia. Unlike the natural selection that has driven the change in the previous two gene sets, sexual selection has driven the increase observed in the number of people carrying genes for this combination of characteristics. Perhaps if Hitler had simply stepped back and let natural selection to take its course, millions of people would have survived what was a brutal and horrifying attempt to ‘fast-forward’ and shape selection during World War 2.

Not only do our modern habits have a dramatic impact on our genome, but modern medicine may also be shaping our evolution in a way that may be more detrimental to the long-term health of our species. Advances in medical science mean that genetic diseases such as type-1 diabetes and haemophilia that previously would have resulted in the death of the sufferer at a young age are now readily treatable. People with diseases once deadly are now able to lead normal lives and survive to reproduce and inevitably, pass on the genes conferring their disease to the next generation. Will we see a rise in these once rare genetic diseases, sentencing our species to a future reliant on modern medicine to survive? Alternatively, will genome engineering see a surge in acceptance? Instead of relying on the slow, unpredictable process of natural selection to shape our species, will we finally be the masters of our own evolution?

Featured image: https://www.sciencemag.org/sites/default/files/styles/article_main_large/public/images/sn-neandertal_2.jpg?itok=co68YoS4