176 New Interpretations: The Neanderthals

Key Idea: New, more complete analysis of Neanderthal DNA is revealing multiple interbreeding events with early humans. Neanderthals appeared about 400,000 years ago, and disappeared 25,000-30,000 years ago. They lived in Europe and parts of western and central Asia. Neanderthals are the

3D closest relative to modern humans, so there is considerable interest in mapping the Neanderthal genome. By comparing the Neanderthal genome to the genome of present-day humans, it may be possible to identify genes in modern humans that have been influenced by positive selection.

Front view

Neanderthal skull Image: Bone clones

La Ferrassie Neanderthal skull, France

NIH DNA being extracted from a Neanderthal fossil

Difficulties in analysing Neanderthal DNA

` The DNA is often degraded to small fragments less than 200 base pairs long. This makes it difficult to obtain sequence overlaps (critical for assembly of the genome). ` The DNA is often of poor quality because it has been chemically modified and degraded by the environment. ` Samples are often contaminated with the DNA of other organisms. Between 95-99% of the DNA obtained from the Neanderthal fossils analysed was from microbes that colonised the bone after the Neanderthal died. Researchers must be careful not to contaminate the sample with their own DNA.

What has been found?

The continuing analysis of Neanderthal (and Denisovan) DNA has found that there were at least five interbreeding events between humans (H. sapiens), Neanderthals, and Denisovans.

` Between archaic humans and Neanderthals. Analysis in 2016 shows there may have been an interbreeding event around 100,000 years ago when an early wave of humans migrating out of Africa met a group of Neanderthals migrating from Europe to Asia. ` Between humans and Neanderthals. Analysis shows that between 1-4% of the genomes of people outside of Africa is derived from Neanderthals (more than for Africans). It is thought these encounters may have occurred as humans migrated out of Africa around 50,000 - 60,000 years ago and met Neanderthal populations already in the Middle East. ` Some human populations that migrated east across Eurasia interbred with the Denisovans. Evidence of this in found in

Melanesian DNA (see opposite). ` Denisovans also interbred with Neanderthals, probably about 50,000 years ago (see opposite). ` Denisovans interbred with an unknown group of hominins, possibly an offshoot of H. erectus, about 100,000 years ago.

The Neanderthal legacy

Analysis of Neanderthal DNA published in 2016 suggests that they carried various mutations that made them up to 40% less reproductively fit than modern humans. When interbreeding occurred with humans, some of these mutations would have been passed to the human gene pool. Over time most of the harmful mutations were discarded through natural selection, but some have remained. Other genes that may have been beneficial also entered the gene pool. However the benefits that these genes once conferred may no longer exist as the human lifestyle becomes more sedentary and diets change. Studies matching health problems to Neanderthal DNA have found that genetic variants inherited from Neanderthals are linked to an increase in the risk of heart attacks, depression, skin disorders, and nicotine addiction. However, the Neanderthal DNA may not necessarily be causing the health problem. It might just be associated with human DNA that is. Some genes that were possibly inherited from Neanderthals or Denisovans have provided benefits. Tibetans appear to have inherited Denisovan genes that enabled high altitude adaptation. Humans may also have inherited genes associated with immunity to new diseases found outside of Africa, but already encountered by Neanderthals.

1. What percentage of Neanderthal DNA is present in modern humans?

2. In which group of modern humans is Neanderthal DNA mostly found and why?

3. Describe some possible positive and negative effects of Neanderthal DNA in modern humans: