Thursday, December 5, 2013

Sequencing the oldest hominin DNA

One of the skeletons found in Sima de los Huesos
assigned to the species Homo heidelbergensis.
Credit: Javier Trueba.  Image source.
In the Atapuerca Mountains of northern Spain, a 43-foot chimney within a cave system leads to a place called Sima de los Huesos, or "Pit of Bones."  Since 1997, more than 5,500 human bones have been pulled from the pit, along with stone tools and the remains of a bear.  Anthropologists consider the human bones to belong to a species called Homo heidelbergensis, thought to be the direct ancestor of modern humans.  H. heidelbergensis is thought to have shared the earth with European Neanderthals, northern Asian Denisovans, and another unknown Asian group.  As mentioned in a previous post, these groups seem to have interbred frequently, such that their distinct species status may not be justified.  In fact, many remains from Sima de los Huesos appear to have some Neanderthal characteristics.

The Internet has been abuzz today with news of the sequencing of DNA from one of the Sima de los Huesos specimens thought to be 400,000 years old, making it the oldest hominin DNA to be sequenced thus far.  The official report was published online yesterday in Nature, but it does require a subscription to access at the moment.  The DNA sequenced in the study comes not from the cellular nucleus, but from the mitochondria, the "power plant" of the cell.  Unlike nuclear DNA, mitochondrial DNA is only inherited from the mother, so any differences between individuals, in theory, must result from mutations, making it very useful in determining breeding patterns and evolutionary relationships.

In this case, the DNA from Sima de los Huesos was compared with DNA from previously sequenced Neanderthal and Denisovan DNA.  The researchers were surprised to find that the H. heidelbergensis DNA showed a closer relationship with the Denisovans than with the Neanderthals, despite having Neanderthal-like physical attributes.  However, this should only surprise those who still view these groups as distinct species.  Because mitochondrial DNA is only passed through the mother, it is entirely possible that the mother had Denisovan ancestry, whereas the father was at least part Neanderthal, reinforcing the concept that these were interbreeding populations of full humans rather than distinct species.

The age of 400,000 years was calculated not from any kind of carbon dating, but from the human mitochondrial molecular clock.  This technique counts the mutation differences between a DNA sample and its modern relatives and produces an age based on the rate of mutation over time.  Mutation rates, however, are difficult (if not impossible) to measure, and they are known to be quite variable.  Therefore, the clock must be "calibrated" using "known" dates of evolutionary divergence, such as the split of humans and chimps, to obtain an average rate of mutation.  As you can see, a long-age evolutionary view is already presumed with this method of dating, so the age is only accurate if the evolutionary model is true in the first place.

From a biblical standpoint, the people found in Sima de los Huesos were, along with their Neanderthal and Denisovan brothers and sisters, fully human beings who were migrating across the face of the earth from Babel around 4200 years ago.  It is true that all of these groups had more robust skeletons than modern humans, but this does not imply evolutionary inferiority.  In the last issue of the Journal of Creation, Dr. Peter Line, a neuroscientist, proposed that the thick bones, high brow ridges, and large braincases found in ancient humans may have been caused by increased thyroid gland activity during development, which may have either caused or been genetically linked to the long lifespans of people before and immediately after the Flood.

I am also tempted to draw a connection from the four people groups (Heidelbergs, Neanderthals, Denisovans, and the unknown group) to the four women on Noah's Ark.  Because mitochondrial DNA is only passed from the mother, we would expect to find four (or three, if Noah's wife bore no more daughters after the Flood) distinct lineages in human mitochondrial DNA.  This is speculation on my part though, but I will keep an eye open for any further evidence that may support or oppose this idea.

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