The role of DNA and mutations in fossil identification
In the past 30 years, scientists have been able to find a record of ancient human migrations in the DNA of living people. The human genetic code, or genome, is 99.9 % identical throughout the world. What's left is the DNA responsible for our individual differences, our eye colour, skin colour eye and disease risk for example, as well as some that serves no apparent function at all. “Once in an evolutionary blue moon”, a random, harmless mutation can occur in one of these functionless stretches, which is then passed down to all of that person's descendants. Generations later, finding that same mutation, or marker, in two people's DNA indicates that they share the same ancestor. By comparing markers in many different populations, scientists can then trace their ancestral connections.
How origins can be traced
In most of the genome, these minute changes are obscured by the genetic reshuffling that takes place each time a mother and father's DNA combine to make a child. However, a couple of bodily regions preserve these tell-tale variations. One, called mitochondrial DNA (mtDNA), which we have already discussed when reviewing molecular dating techniques, is passed down intact from mother to child. Similarly, most of the y chromosome, which determines maleness, travels intact from father to son.
The accumulated mutations in females’ mtDNA and, for males, the Y chromosome are only two threads in a vast tapestry of people who have contributed to your genome. But by comparing the mtDNA and y chromosomes of people from various populations, geneticists can get a rough idea of where and when those groups parted ways in the great migrations around the planet.
A comparison of mtDNA from women around the world has found that women of African descent showed twice as much diversity as their sisters. Therefore, since the tell-tale mutations seem to occur at a steady rate, modern humans must have lived in Africa twice as long as anywhere else. Scientists now calculate that all living humans are related to a single woman who lived roughly 150,000 years ago in Africa - a "mitochondrial Eve." She was not the only woman alive at the time, but if geneticists are right, all of humanity is linked to Eve through an unbroken chain of mothers.
Mitochondrial Eve was soon joined by "y chromosome Adam," an analogous father of us all, also from Africa. Increasingly refined DNA studies have confirmed this opening chapter of our story over and over: All the variously shaped and shaded people of Earth trace their ancestry to African hunter-gatherers.
For example, in 2010 the skeleton of a man who lived more than 2,300 years ago was found in St Helens Bay off southern Africa. Mitochondrial DNA established that he was a hunter gatherer who foraged for food in the ocean and came from a lineage, now extinct, which diverged from the original Mitochondrial Eve, who lived in Africa about 200,000 years ago (sic), about 150,000 to 170,000 years ago. Sydney Professor Vanessa Hayes was able to generate a complete mitochondrial genome of the man using material from a tooth and a rib. In 2010, Professor Hayes, an expert in African genomes, sequenced the genomes of four Kalahari Desert bushmen, which showed that the hunter-gatherer bushmen of southern Africa have remained isolated from other populations for about 100,000 years. While this group were around at the same time as the St Helens man and other coastal dwellers like him, the two groups remained isolated from each other. Genetic studies of modern Kalahari bushmen show that they do not share the man’s unique mutations[1].
The Sima de los Huesos fossils [2]
The different outcomes which can be generated by nuclear DNA and MtDNA analysis is illustrated by the case of 28 hominin fossils from the Sima de los Huesos (“pit of bones”) site in the Atapuerca Mountains Spain. They were found back in the 1990s but scientists still haven’t been able to settle on their genetic origin. Recently, by managing to extract some truly ancient nuclear DNA, researchers came to the view that the 430,000 year old Sima bones were Neanderthal in origin - there were also some striking physiological similarities - or perhaps they belonged to the species Homo heidelbergensis, which lived in the right place and the right time and was thought to be an ancestor of the Neanderthals.
However, MtDNA analysis in 2013 - MtDNA is more easily recoverable from ancient bones than nuclear DNA - suggested that the Sima hominins were actually more closely related to the Denisovans - and here's the rub, yet it also shows that the nuclear DNA examined is consistent with that you would expect to find in a Neanderthal ancestor! It should also be noted that the divergence between Neanderthals and Denisovans occurred more than 430,000 years ago. In other words, we are left with a tangled web of apparently interwoven early human relationships - if that is indeed what they are - but as Rachel Feltman concludes, the DNA analyses of these primitive early species shows just how far research techniques have come in only a few decades, even if they do also happen to throw a few surprises into the ring in so doing. [3]
[1] Nicky Phillips, “Skeleton found in South Africa comes from earliest ethnic group of humans”, SMH, 1 October 2014.
[2] Rachel Feltman, "Pit of bones could hold the keys to family split", SMH, March 19-20, 2016, extracted from the Washington Post. See also https://en.wikipedia.org/wiki/Atapuerca_Mountains#Sima_de_los_Huesos_.281983-.29
[3] A replica skull, said to be 300,000 – 500,000 years old, then attributed to Homo heidelbergensis from the Sima de los Huesos, Sierra de Atapuerca site, may be found in the Dublin Museum of Natural History and is shown in Appendix 4 at /appendix-4---dublin-museum-of-natural-history.html