The problems associated with reconstructing the evolution of Homo sapiens

By Sarah Aldridge

There is a natural desire to understand our own ancestry; to understand from where Homo sapiens originated and evolved, how we spread from those origins and how we evolved further as we adapted to occupy every corner of the Earth.

But this task is not an easy one, a number of obstacles remain in our way. Problems encountered in this immense task include an incomplete fossil record, the absence of any other extant Homo species, but the presence of their DNA in our own genomes, and the continuing argument of just how we should be categorising these species, and how we can distinguish between species when the differences within our own species are so pronounced.

The most widely accepted present theory is that our species originated in Africa and spread across the rest of the world from that location1. The specifics of how this expansion progressed are a topic that is still widely debated, in particular, the details of our interactions with other Homo species: the Neanderthals, the Denisovens, and a third currently unknown species.

The Theories.

The previously accepted theory known as the multiregional (or regional continental) hypothesis theorised that a Homo erectus dispersed across the world, and evolved separately in each region, but continued to interbreed. This theory accounted for the regional diversity found among Homo sapiens.

The more current theory is known as the Replacement (or Out of Africa) Model, this theorised that an archaic hominid spread worldwide, but only evolved to become Homo sapiens in Southern Africa (approximately 100 000 years ago). From here, the species then spread from this one location around 60 000 years ago and interbred with other hominid evolutions before their eradication. As a result, many people living outside of Africa have inherited small but significant amounts of DNA from these extinct, archaic humans 2.Image

[Our evolutionary history displayed across two continents over 2 million years.]

The problems with the reconstruction.

Defining a human

One problem that is frequently visited upon, is exactly how to you define a modern human? All living humans are considered to fall under this category, but the earliest definition of a species that anyone learns is that two different species cannot interbreed to produce fertile offspring. If this basic law is observed, then both modern and several archaic forms of Homo should be considered to fall under the one species name. This topic is still debated, but for the sake of this post, we shall continue with the naming demonstrated in the figure above.

Once we’ve decided on how to define the species on paper, the next obstacle is to identify the individual species, and determine the variation within it. It’s easy to see a huge range of morphological differences between individuals within Homo sapiens, these regional features occur because they have evolved independently as pioneer populations adapted to these new areas. These features can include facial shape, the forms of eyelids, type of hair, differences in skin pigmentation and in physique, all of which vary from region to region, and individual to individual. These factors are so pronounced, that just by looking at the skeletons different races, sexes and ages can often be distinguished.

With such massive variation in ourselves, how can we expect to tell the difference between our own skeleton, and those of other Homo species? Well, there are a number of features that help with this morphological identification. For one, the skeleton of a Homo sapiens is much more gracile than that of other species in the genus. That is, that our skeleton is more slender and fragile, but much lighter too, with differently proportioned long bones, and less pronounced muscle indentations. There are a number of differences that can be demonstrated in the skull as well, including the widely known Neanderthal brow ridge (see image below)3. In addition to these visual techniques, it has more recently become possible to compare them via their DNA as well.

Image

[The distinguishing features between a human and a Neanderthal skull, note that each further variation between skulls within each species not described here.]

What were our interactions with other species?

It is known that modern humans and Neanderthals overlapped in their time and spacial distributions, from this we know there must have been some form of interactions, be it fighting, interbreeding or simply cohabiting the same land. We know from our own genetic evidence that there was indeed interbreeding taking place upon leaving Africa, similarly observed with the Denisovens (whose DNA is present in individuals found Australasia and Papua New Guinea), and a third currently unknown species from Sub Saharan Africa4.

But what happened to these other hominids? Why aren’t they still coexisting with us today? Why have we outlasted the Neanderthals, who disappeared 30 000 years ago after a 200 000 year existence? The answers to these questions are no fully known, but a number of hypotheses have been put forward. The climate around that time was changing and fluctuating rapidly, for which they may not have been able to cope because of their energetically expensive structures, or merely another part of a megafaunal extinction. It’s possible that disease could have driven them to extinction, or competition of resources with our own species, or even conflict. It is also feasible though, that the Neanderthals never really went extinct, but were simply genetically absorbed into our own ancestors through interbreeding.

Conclusion

While these obstacles have proved challenging, it is by overcoming them that we better understand our origins. By genetic testing we have discovered that we are a hybridisation of multiple ancestors, and that this, in combination with the Out of Africa theory, is the best suited model for our existence.

But this is not a complete model, and further research needs to be done to increase the accuracy of the chronological frameworks of our expansion, and map our progression across the globe more accurately, better observing the different pioneer populations between continents.

References

[1] Online resource <http://anthro.palomar.edu/homo2/mod_homo_4.htm&gt; [date accessed 30/11/2013]

[2]Henn B.M., Cavalli-Sforza L.L., Feldman M.W. (2012) The great human expansion, PNAS109, 17758-17764.

[3]Stringer and Groote (2013) Homo sapiens. In: C. Smith (2013) Encyclopaedia of Global Archaeology, New York, Springer Science + Business Media New York, p1-7.

[4]Stringer (2012) What makes a modern human? Nature485, 33 – 35.

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