Saturday 2 July 2011

Out of Africa(?)

There are currently two main theories about the evolution and global migration of modern man.

The Out of Africa Hypothesis postulates that modern man evolved in Africa, with the first true members of our species appearing in Africa between 500 000 and 200 000 years ago, and then migrating out of the continent some time after 125 000 years ago, replacing all other species of humans.

The Out of Africa Model.

The Muli-Regional Hypothesis postulates that various species of humans spread around the globe about 2 million years ago, and that these separate species evolved into modern races of humans.

Different theories are part of the normal run of science, but when human origins are involved they can become very political. The two theories have very different implications as to how closely related we all are. Either we all share a common ancestry within the last half a million years and racial differences are merely superficial local adaptations to climate, or the different modern races are so distantly related they are nearly different species. However we cannot dismiss a scientific theory because we do not like its implications, but should consider the evidence for and against rival theories and try to make decisions based upon this.

The Out of Africa theory has the backing of the majority of palaeoanthropologists (scientists who study human origins). It was initially proposed by Charles Darwin in his 1871 book Descent of Man; a single human origin fitted with his theory of natural selection so this part of the theory is not overly surprising, what is surprising is that he postulated an African origin for mankind. At the time most scholars placed the origin of man in the Middle East, based upon biblical scholarship, the best evidence they were aware of (fossil humans were more-or-less unknown at this time). Darwin based his ideas on the African origin of man upon his (new and controversial) idea that we are descended from African apes. This was still at best a guess; the natural world was not as well documented in the eighteenth century as it is now, so it would not have been a great surprise if more human-like apes had subsequently been found somewhere in the jungles of Asia.

Charles Darwin, who's controversial views on the origin of man have stood the test of time.

The Multi-Regional Hypothesis as we currently know it did not exist in the eighteenth century, but it had its parallels. In the eighteenth century it was generally assumed that the different races of men were different species. When Linnaeus described humans he gave the different races different specific names; Homo sapiens (the thinking man) only applied to Europeans. This was convenient politically. It could be assumed that the biblical tales were about white people, when Jesus spoke of love for ones fellow man he obviously meant white men. Scholars seriously argued that Africans did not actually have souls; whatever the case it was clearly OK to enslave them; either they were animals, or an inferior sort of humans placed on Earth by god to do manual labour for white people. When Jean-Baptiste LaMark proposed his theory of evolution in 2009 he did not go as far as to say that different humans were different species, but his theories could definitely be read that way. LaMark thought that animals evolved by striving to improve themselves, actively trying to evolve towards a platonic ideal. Obviously in this scheme of things humans (and European humans at that) were the pinnacle of evolution, to which all other animals would strive. This could be interpreted to mean that different groups of humans had completely different origins. This theory did not even imply that apes were more closely related to humans than clams; species were more similar to humans because they were more strong willed than other species.

Jean-Baptiste LaMark's ideas on evolution seem ridiculous now, but were advanced for his time.

Since Darwin's time many fossil humans have been found, and a variety of human species named and placed in a number of genera. Most of these have been restricted entirely to Africa, but other members of the modern genus Homo have also been found in Asia and Europe. Nobody any longer disputes that the first humans arose in Africa, but it is disputed how long ago the ancestors of modern humans left Africa and diverged from one another.

Scientists have studied the genes and chromosomes of modern humans and come to some remarkable conclusions. Chromosomes have large chunks of non-coding DNA (sometimes called junk DNA). This DNA does not produce any proteins, so if it undergoes mutations this does not have any effect, good or bad. So while coding DNA (the DNA which codes for proteins) is subject to natural selection - beneficial mutations will quickly spread through a population, harmful ones will quickly be eliminated - non-coding DNA is not. Mutations can therefore accumulate at a steady rate, and this rate can be used to judge the time since different lineages diverged. This is known as the molecular clock.

Chromosomes come in pairs, and in each generation the genes on those pairs are recombined, forming a new chromosome with a combination of DNA from each parent, this can make the molecular clock method hard to interpret. However humans (and all mammals) have two sets of DNA which do not recombine; the male sex chromosomes Y, which is passed from father to son and never recombines with its partner the X chromosome (X chromosomes do recombine in women, so these are less useful), and mitochondrial DNA; DNA found outside the nucleus in organelles called mitochondria, and which are always inherited through the female line. It is therefore easy to assess the rate at which these chromosomes have accumulated mutations, making them a prime target for scientists interested in tracing the divergence of human populations.

When scientists examined the X chromosome they came to the conclusion that all humans had a common female ancestor approximately 160 000 years ago. This hypothetical female ancestor is sometimes known as the 'Mitochondrial Eve'. The Y-chromosomal DNA yielded even more surprising evidence, all male humans apparently shared a single male ancestor 60 000 years ago, sometimes called the 'Y-chromosomal Adam'.

This was not the end of the DNA story. Scientists were also able to analyze the entire human genome to look for diversity within different groups. By analyzing the DNA of thousands of volunteers from around the world it was possible to build up a rough family tree for humanity. This suggested that the greatest human diversity was found within African populations - all non-African populations, no matter what they look like - are comparatively closely related to one-another.

A family tree for the human race, based upon mitochondrial DNA

Evidence for the Multi-regional hypothesis is less solid. Some palaeoanthropologists claim that they can discern features on the skulls of ancient humans which resemble modern humans living in the same areas, and there have been claims (dubious) claims of recovered DNA from Neanderthals which suggests they might share some genes with modern Europeans which other ethnic groups lack.

Nevertheless the popular press has taken quite a liking to the Multi-Regional Hypothesis, and it is often reported as being of equal merit too, or even favored over, the Out of Africa Hypothesis. So when stories started appearing in the British media this week of a new paper being published that cast further doubts upon the Out of Africa Hypothesis, I was immediately suspicious.

Doubts over 'out of Africa' theory, Yahoo News, 29 June 2011.
'Out of Africa' theory thrown into doubt, Sunday Mercury, 30 June 2011.

The paper in question appeared in the journal Plos One and was by a team lead by Etty Indriati of the Laboratory of Bio- and Paleoanthropology at the Faculty of Medicine at Gadjah Mada University in Yogyakarta, Indonesia.

It does not actually make any reference to either the Out of Africa Hypothesis, or the Multi-Regional Hypothesis, but provides a re-evaluation of the date of the 20 Meter Solo River Terrace, one of the 'Java Man' sites, where remains of the the presumed human ancestor Homo erectus was found. Homo erectus has been found in eastern Africa, Georgia in southeast Europe, Turkey, India, China, Vietnam and Java, though not all scientists accept that all these specimens belonged to the same species. Most of these finds are over a million years old, but a much younger date, possibly as recent as 35 000 years ago, has been suggested for the Solo River site.

A reconstruction of 'Java Man'; the dark part on the skull was actually discovered, everything else is speculation.

What Indriati et al. did was to re-date the sight of the Solo River finds using isotope analysis of ash layers. This works something like this:

Mineral crystals are formed in volcanoes as liquid magma cools and solidifies. Each mineral forms at a range of temperatures, and (importantly) contains known elements. Some of these elements are mildly radio-active; they decay over time to form new elements. Since these new elements have different chemical properties, they would not have been present in the original minerals. Thus, since the rate at which these elements decay is constant, it is possible to compare the ratio of the original element to the product element within a mineral crystal and produce a date for the rock.

The Solo River site is on a floodplane, the material which makes it up has been deposited by rivers, and shows some mixing of material. However horizons of volcanic material exist within it that are unmixed, they give a constant date reading, implying they were deposited by discrete volcanic episodes. Using these horizons Indriati et al. were able to come up with a date of slightly over half a million years for the sight.

A reconstruction of the Solo River site.

What this tells us is that Java man was not a contemporary, or near contemporary, of the earliest modern humans in the area, or of Neanderthal man in Europe, as has been suggested, but rather lived and died long before the earliest anatomically modern humans appeared in Africa (or anywhere else). This does not preclude the possibility of Homo erectus having lived in the area until much more recently, but it does leave us with no evidence of this. As such it tends to support, rather than cast doubt upon, the Out of Africa theory.