Our Ancestors Were Not Fussy about Partners

Modern humans and Neandertals interbred multiple times…

A new series of genetic studies show that ancestors of modern humans, now-extinct Neandertals and Denisovans, another ancient human species, exchanged genes through at least five episodes of interbreeding over the past 60,000 years and that present-day humans carry this mixed genetic heritage with the exception of Africans whose ancestors never met with the Neandertals. In another study, the modern human DNA identified in a fossil Neandertal toe bone provided evidence for yet another instance of breeding between the two human species about 100,000 years ago. In still another study, researchers have dug out proof of similar intercourse between the ancestors of Neandertals and Denisovans. Studies also reveal exchange of diseases between the Neandertals and modern humans in the course of interbreeding. Meanwhile a study found the Neandertal Y chromosome to be incompatible with modern human genome, which means Neandertals could not father male children from female humans, a fact which might have hastened their disappearance.

Genes fly in the air!

Findings of an international team of researchers published in Science show that the family tree of East Asians contains genes of three different Neandertals resulting from breeding at separate occasions. Neandertal gene injection to Europeans and South Asians was made twice and to Melanesians once. Meanwhile, genetic fingerprints of at least one mating between Denisovans and Melanesians have been identified.

According to Svante Pääbo of the Max Planck Institute of Evolutionary Anthropology in Germany who had led the team which first sequenced the Neandertal genome, Europeans and Asians have inherited an average 1.5 percent of their genes from the Neandertals. Melanesians, meanwhile owe another 2-3 percent of their genome to Denisovans in addition to the Neandertal portion.

The cost of mixing

Various studies show that as a result of these matings, especially the Neandertals and modern humans could have exchanged certain diseases to which they were exposed and developed resistance in their original habitats. 

According to a study authored by Dr Charlotte Houldcroft of the Biological Anthropology Department of Cambridge University and Dr Simon Underdown, published in the American Journal of Physical Anthropology , modern humans who acquired theulcer-causing bacterium Heliobacter pylori from a more primitive hominin – who, in  turn got it from chimpanzees – in Africa before migrating to Europe and Western Asia , passed it to hunter-gatherer Neandertals there living as bands of 20-30 people, thus possibly contributing to their sudden departure from the stage. Besides stomach ulcers, other diseases and pathogens which modern humans passed to the Neandertals, disrupting their health , reducing their ability to find food and ultimately leading to their extinction, were flatworms of African origin, tuberculosis and herpes strains contracted through sexual intercourse or contact with saliva.

In exchange, modern humans received from Neandertals certain genetic features which had propped them up in their European habitats. These include resistance against sepsis and encephalitis.

A study by population geneticist Joshua Akey and  evolutionary genomicist Tony Capra of the Vanderbilt University also points to Neandertals as the source of some genetical features, some beneficial and others harmful, in modern humans.

A gene variant the modern human ancestors inherited from Neandertals makes the blood more viscous and facilitates coagulation. It is clear that this genetic feature has benefited the Neandertals in coping with injuries sustained when hunting dangerous animals and haemorrhages while giving birth to babies with large brains. But at the same time, it increases the risk of clotting and strokes. Depression linked to disrupted circadian rythms and pre-cancerous skin lesions called actinic keratosis in modern humans are also among the heritage of mating with Neandertals. Genes regulating the brain chemistry and skin responses of the Neandertals attuned to natural light conditions and lifestyles in Europe, also have negative effects on modern day humans under living and working conditions based on artificial lighting.

One Neandertal gene allele regulates the transport of thiamine (vitamin B 1) which metabolizes carbohydrates in gut cells. Neandertal diet, rich in meat and nuts, produce an adequate supply of thiamine. But modern-day diets based on processed food do not provide enough thiamine, leaving those with the Neandertal allele susceptible to malnutrition.

Researchers link such conditions as incontinence, bladder pain and some urinary tract disorders with Neandertal genes. An allele associated with tobacco addiction has also been revealed to be a gift from the Neandertals.

When modern human ancestors arrived  at Europe and the Middle East where Neandertals were living from 200,000 years back and had evolved defense mechanisms against pathogens in their natural habitat, they took over a a stronger immune system from the residents, defending the body against bacteria, fungi and parasites. Three genes inherited from the Neandertals are thought to beef up the body’s defense system by coding for certain receptors on white blood cells, the main fighters of the immune system.

But while this reinforced immune system has come as a boon for the people of ancient times as well as those living in poorer areas today, it is also a chief cause of allergies in developed countries where parasites are relatively rare.

Boys paid the price!

An asset the Neandertals might have been particularly stingy in handing down to modern humans is boys. A study led by researchers from Stanford University School of Medicine on samples taken from a 49,000-year-old male Neandertal fossil found in Spain has shown that genes on the Neandertal Y chromosome differ from those on the same chromosome of modern humans. Researchers conclude that Neandertals might have carried genes on their Y chromosomes which were incompatible with other human genes. These gene variants suggest Neandertals could have had problems in producing sperm and that Neandertal males and modern human females could have been sexually mismatched. On the Y chromosome of the fossil, researchers identified mutations in three of the immune genes. One of these codes for an antigen which triggers an immune response in pregnant women, causing them to miscarry male fetuses carrying these genes. That means that despite numerous episodes of interbreeding throughout the history, these two human species could not have too many boys from these matings.