Oh dear....

Trawling through the internet this evening I stumbled across this article. From real scientific theories to hilarious claims of sequencing Bigfoot DNA. This is the kind of thing to really not fall for (unless you're having a very silly moment)!

The geographic review of Homo sapien - An update!

This post is a follow on to the review posted on Friday the 9th of November (you might want to brush up a bit before you read on so I don't leave you behind). In our first look into where on earth our species comes from I took you on a tour through the groundbreaking primary papers which first furnished the theories with scientific evidence. However as is always the case with science, things move on rapidly; new research comes out all the time, slowly allowing us to sift out all the falsities thereby allowing the thousands of year old tale to be told.

When I last left you it was with the notion that the best supported view of the scientific community was that of the Out of Africa hypothesis. The Multiregional theory being blown out of the water by mitochondrial DNA evidence. However new research muddies this clear distinction (things in science are never as simple as they seem).

Recent scientific advancements now allow us to sequence the genomes of the most recent species of Hominin we shared the world with. These include the Neanderthals and the Denisovians. The results of these studies were published by Green et al. and Reich et al. respectively in 2010. The findings of which are discussed by Svante Paabo in the video below.

The Out of Africa hypothesis is defined as a total replacement of the local populations by Homo sapien with NO interbreeding, whereas the Multiregional hypothesis proposes an independent evolution of modern from indigenous Hominin species. The new research suggests a compromise between the two extremes; H.sapiens did emerge from Africa to conquer the world but they got jiggy with the locals too, and in doing so enabled the preservation of an extinct species to live on in us today .

Figure 1 H. sapien falls for a Neanderthal

This all seems fine and dandy, however calculating these findings is error prone. From information gained in previous molecular biology modules I understand there to be many complexities and problems encountered sequencing ancient DNA; in some instances the DNA is contaminated and the strands being sequenced are in actual fact the genetic material of the researcher. The two studies mentioned above have undergone rigorous peer reviewing and therefore the results are much more reliable, for the time being it is to be believed that Europeans share 1-4% of DNA with Neanderthal man and Melanesians share 4-6% of DNA with Denisovans; the question is how Neanderthal are you?

Climate's role in Out of Africa 1

So now we've explored the route the first adventures took, lets take a closer look at what was going on with the climate. The reason why I'm devoting a blog post on this topic is that the changes in climate in the time period 1.8-2 ma (million years ago)  was one of the main factors enabling this migration of Homo erectus to occur.

The epoch we are looking at is the Pleistocene, it lasted from approximately 2.6 ma until 11,000 years ago and is characterised by a climate which fluctuated between cold glacial to warmer inter glacial periods. As the world slipped into a glacial period, huge ice sheets would descend from the poles and drape themselves across the Northern Hemisphere, locking away massive quantities of fresh water in the process.

Figure 1 Although it doesn't go right back to the period we're looking at this graph gives you an idea of the shifting climate occurring in the Pleistocene

Prior to the Pleistocene the climate had been relatively stable; in Africa grass lands were dominant and the rainforests had receded. It is interesting to note that this change can be determined by looking at the type of carbon that has been stored by the plants - C4 grasses are more dominant in arid environments and C3 leaves and plants prefer warm wet conditions. We use carbon dating amongst other methods to create a picture of the palaeoenvironment.

So anyway, the stage was now set for migration: resources were becoming scarcer, resulting in groups of Hominins having to travel further for their meals. There was only one problem - there was no way out of Africa. This all changed with the arrival of the glacial periods; the formation of ice requires large quantities of water, water which was sourced from the oceans. So much water was taken to form these ice sheets that sea level dropped by around 100m, opening up land bridges and allowing species to cross continents. In our case this land bridge was the Levantine corridor in Israel. Once H. erectus were across this land bridge they could spread onwards to conquour the rest of Asia and eventually a very chilly Europe.

Figure 2 Hominins in Europe

Video Time 2

Hey guys thought I'd take a break from all the serious long posts by giving you a bit of light relief in video form.

If you haven't heard of it before The Incredible Human Journey is great! Skip to episode 2 for a thorough analysis of the Multiregional vs Out of Africa debate I was explaining in a previous post.

Out of Africa 1 - Homo erectus dispersal

When reading my last blog post on the geographic origin of Homo sapiens it may have surprised you to learn that our own species was not the first to venture out of the motherland and into unchartered territories. The first colonisation was actually achieved over a million years before our H. sapiens had even evolved. This ancient species began as H. ergaster and became H. eretus

The origin and dispersal of these species is still much debated (anthropologists never seem to agree on anything!), but I will take you on the journey that is most widely accepted. If you are interested in other theories then please comment below and we can examine the debate in detail.

The setting today is Eastern Africa 1.8 million years ago – the home of H. ergaster. This species possessed a larger brain than its predecessor H. habilis, and was likely fully bipedal; the remains of this species is almost entirely found in the African continent. However it seems these apemen were not content in their home land as a small band managed to migrate north across a grassy Sahara and across the Levantine corridor into Israel. It is in this region of the world that we think the speciation into H.erectus occured as speciemens have been found in Dmanisi Georgia. This new species was the most advanced Hominin yet and had many similarities to us. However there was one very noticeable difference - these guys had HUGE brow ridges; the purpose of which is still unknown.

Take a look at those brow ridges! Fig 1

 Their journey continued from here, over the next few 100,000 years they spread across Asia and into Europe; we know this by analysing remains which have been found in S.E Asia 1.6ma and Spain 1.1ma.

The route taken by H. erectus Fig 2

To be able to master the multitude of ecosystems that must have encountered along their epic journey across the Old World they must have been highly adaptable and extremely hardy. Their success may be owed to their advanced stone tools made with Archeulean technology and their highly meat orientated diet, allowing enough energy to fuel a big brain. Interestingly none of these tools are found east of India, a cut-off named the Movius Line. However in my opinion this is due to H. erectus's adaption to utilising local materials such as bamboo to make tools.

I hope you’ve learned something in this post, next time we’ll examine how climate enabled this migration to take place. 

The geographic origin of Homo sapien. A review

As promised this post will be another literary review of key papers which shape our understanding of human origins. Today we are going to examine opposing theories which attempt to explain how our species Homo sapien developed from the ancestral condition.

First let us paint a picture of the time period we are examining. It is 200,000 years ago; different species of the genus Homo are already dispersed all over the Old World (Africa, Asia and Europe): with H. erectus in Asia and Neanderthals in Europe. The climate is fluctuating between cold glacial and warmer interglacial periods and mega fauna dominate. This is the time where Homo sapiens make their debut appearance. Papers propose differing views of how this development into modern man came about; did H. sapiens evolve in Africa and replace all existing Homo species, or did they arise independently from pre existing local populations?

Anthropologist Allen Wilson was a proponent of the former view, the ‘Out of Africa’ hypothesis. In his 1987 paper he provides supporting evidence by analysing the mitochondrial DNA of different human populations from around the world. I realise many of you may not know what mitochondrial DNA is so I will give you a brief description: mitochondria exist in every cell in your body; they were once bacteria and act as oxygen factories. Mitochondria have a genome separate to the nuclear genome (chromosomes) we all know and love.  The brilliant thing about the mitochondrial genome is that they do not recombine like our chromosomes and instead passes intact directly from mother to offspring.  This allows the mitochondrial genome to be tracked back in time! Wilson chose 5 different populations from around the world: Africans, aboriginal Australians, Caucasians, Asians and aboriginal New Guineans and used their mitochondrial DNA to produce a restriction map. This map identifies the mutations which have occurred in each lineage, and upon comparison to one another can ultimately show where and when the original ancestor of all lineages existed. The conclusions made by Wilson are that this Eve of humanity existed about 200,000 years ago in Africa.

The consequence of this conclusion is that Homo sapiens must have evolved in Africa before spreading fully formed (with a brain size identical to our own) to colonise the rest of the world. Any species they may have met on their travels be it H. erectus or Neanderthals would have been outcompeted and driven to extinction.

Figure 1 A diagram of the two theories

The suggestions made by Wilson’s paper were controversial and were quickly attacked by proponents of a multi regional human evolution. A paper by Thorne and Wolpoff in 1992 argued that the methods used to calculate the 200,000 year old date of divergence was inaccurate. They suggested that the date of divergence of these 5 human populations should have actually been 1 million years ago. If this were true then the scenario would be very different: the original ancestor would have been much more primitive with a smaller brain size, each different regional population would have developed into a modern human separately and in situ. In Asia H. erectus would have developed into modern Asians and In Europe H. erectus would have developed into Neanderthals and then into modern Europeans; with interbreeding happening along the way.

Although this idea does seem quite outlandish Thorne and Wolpoff do provide some good evidence; the technology and culture of modern humans in Europe were identical to the Neanderthals they lived alongside including their burial systems and butchering of game. If Wilson’s idea were correct then it would be expected that the hand axe technologies of the African Eve population would spread alongside their population, however no hand axes are found outside of Africa. These two examples show that either the Out of Africa theory is incorrect or that the modern humans leaving Africa adapted to local practices and given up any former technology they had developed. 

Another important disparity discussed by Thorne ad Wolpoff’s paper is the lack of African cranial features found in Asia and Europe. Instead certain diagnostic characteristics of populations continue right up until the present day. Although the paper includes many examples, the best one comes from examination of Chinese fossils. The unique feature of a shovel like maxillary incisor is shared by both ancient specimens and the modern population of China; supporting an in situ transition from ancient to modern local population and not a replacement.

All of the evidence provided by the multi regional hypothesis is quite convincing, however the majority of the scientific community still support the Out of Africa theory. I believe this is mainly due to the lack of substantial fossil evidence to really support Thorne and Wolpoff’s claims. On top of this using mitochondrial DNA  to calibrate an ancestors age is a method approved by many to be accurate enough to use as evidence, and therefore the claim that Wilson’s method is inaccurate does not stand up to scrutiny. As is usually the case in a scientific debate more evidence is required to determine which paper is valid but I hope you have gained enough information from this review to form your own opinion on this topic.  

This review has been about the classic papers, for a more up to date analysis check out my updated version (30th November)..

References

Thorne, A.G., and Wolpoff, M.H., 1992. The Multiregional Evolution of Humans. Scientific American. 266(4) pp. 28-33.

Cann, R.L., Stoneking, M., and Wilson, A.C., 1987. Mitochondrial DNA and human evolution. Nature. 325(1) pp. 31-36.

Amazing summary!

This is a fantastic summary of everything mentioned so far, the guy presenting it is incredibly engaging, if you're interested in sciencey stuff I recommend you watch more of his videos.

Also another review is being posted in the next couple of days, so come back again soon!

I've just been reading an interesting article on the BBC website published last year. It suggests that human origins can be traced back to southern Africa using studies of genetic variation. You can view it here; let me know your views on it!