Multiregional or Replacement?

There are two main models which attempt to explain the emergence of the modern human, how they colonised the world and outlived the archaic humans. These are the multiregional model and the replacement model.

Homo erectus 

The Multiregional Model

The multiregional model suggests that Homo sapiens didn't evolve at a specific time or place, instead they slowly evolved from Homo erectus (right) an ancestor of the modern human, that lived between 800,000 and 1.8million years ago, all over the world. Because they lived all over the world, gene flow between the geographical regions would have been constant. The multiregional model also suggests that interbreeding occurred between the Homo erectus and different types of archaic humans. And eventually, over many hundred thousands of years, all over the world, the Homo erectus became the modern human.

But how can this be possible? The earth is a large place, the chances that several different populations from several parts of the world, evolved in ‘exactly’ the same way at ‘exactly’ the same time is about as close to impossible as you can get. This unlikely type of evolution is known as ‘convergent evolution’, one of the main reasons the multiregional model is opposed by the replacement model.

The Replacement Model

The replacement model agrees that the Homo sapiens evolved from the Homo erectus, however, the replacement model believes that some populations of Homo erectus, migrated out of Africa and spread throughout the world. Those that migrated out evolved in a different way to those that stayed behind in Africa. The Homo erectus that stayed behind in Africa evolved into the modern humans, the Homo erectus in Europe evolved into the Neanderthals and the Homo erectus in Asia evolved into Denisovans. Another migration then occurred around 50,000 years ago, where the modern humans from Africa spread to Asia and Europe, replacing the archaic humans there, without interbreeding. The replacement model believes that at some point modern humans, Neanderthals and Denisovans cohabited in some parts of the world.

Although the replacement model is the leading and most widely accepted theory of the origin of the modern human, more recent genetic evidence shows that modern humans DID in fact interbreed with the European Neanderthal and the Asian Denisovans. So which is it?

Finding Eve: The Cartoon

Mitochondrial Eve?!

Whilst doing some research for my dissertation piece, I stumbled upon a very interesting topic about a woman; mitochondrial Eve.
According to the experts in human evolutionary genetics, Mitochondrial eve is the matrilineal, most recent common ancestor of modern humans! To you and me, this simply means that Mitochondrial Eve is our great, great, great, greatest, even greater than that, grandmother.

So how did the experts figure this out? Mitochondrial DNA is passed from mother to offspring without recombination, so therefore, all mitochondrial DNA in every living person is exactly identical to their mother's, and her mother's mother, and so on. So therefore we can use mitochondrial DNA to trace back the earliest of modern humans. Mitochondrial Eve is estimated to have lived around 160,000 to 200,000 years ago, in East Africa. Unlike the Biblical Eve, Mitochondrial eve was not the first and only woman alive during her time, however, her offspring are the only ones who made it into modern humanity, meaning the offspring of the other women of her time died off at some point. e.g. by only having male offspring that could not pass on the genetic information, or by dying through natural causes before reproducing. Times were tough back then. (I imagine). It was the discovery of mitochondrial Eve that made the hypothesis that modern humans originated recently from Africa and then spread from Africa to the rest of the world, replacing the archaic human populations that existed at the time, such as the neanderthals, the predominant and leading theory supporting our origin.

Y - Chromosomal AdamMitochondrial Eve is not our only most recent common ancestor! Meet Y-Chromosomal Adam. The Y chromosome is another part of the genome that is not shuffled about by evolutionary mechanisms in order to generate diversity. This means that the male Y chromosome is passed on in tact, so therefore Y chromosomal Adam is our great, great, most greatest, even greater than that grandfather. Y-chromosomal Adam is estimated to have lived also in Africa but at a different time to Eve, 140,000 years ago. Using mitochondrial DNA analysis we can look back on the genetic ancestry of the modern human like never before.

In my next post, I will be explaining the four main thoeries which try to explain the origin of the modern human.

Journal Review: Why did modern human populations disperse from Africa 60,000 years ago?

This post is based on a very enlightening paper I came across in the Joural "Proceedings of the National Academy of Sciences", written by Paul Mellars in 2006.

The paper 'Why did modern human populations disperse from Africa 60,000 years ago? helped to confirm the current beliefs on the origins of the modern human. Modern humans, are those who are anatomically and genetically identical to humans today and research shows that these modern humans emerged from Africa up to 200,000 years ago.

However, around 65,000 B.P (before present), a major dispersal towards Asia and Europe occured. Research in this department tends to use genetic methods, using mitochondrial DNA (mtDNA) analysis and comparison, matched with archeological findings. These findiNgs suggest a sudden increase in the complexicity of technological, economic, social and cognitive behaviour of certain African groups which could have led to their demographic expansion due to competition with one another.The paper further suggests that it is the development of advanced behavioural changes that could have led to the expansion of the L2 and L3 mitochondrial lineages over Africa, to Asia and Europe. The behavioural changes may have also helped these modern humans to replace the 'archaic' humans that previously occupied these regions, with or without interbreeding.

The paper shows what the science community now knows about the mdoern human...DNA patterning of present world populations and archeological findings of skeletal remains shows that populations that were genetically and anatomically 'modern' emerged from Africa up to 50,000 years ago. These populationas dispersed from Africa to other parts of the world around 40,000n years ago, where they replaced the pre-existing 'archaic' populations there, such as the European Neanderthals.

The paper raises the question of why it took the modern human up to 100,000 years to disperse from Africa, and what are the crucial evolutionary and adaptive developmets tahta allowed these populations to colonise and defeat a range of entirely different environemts and its inhabitants. The paper further suggests that the answer to these questions lies partly in the results of recent DNA analysis among different geographical groups of Africans of the present day.
In my next post, I will be looking further into key discoveries that have been made over the past 2 decades that helped form our current understanding of where we came from.

What is it about Nigeria that makes me have brown skin?

When I began my initial research into 'where do we come from?', I was confused about exactly what this meant. There are many theories which explain our origin, including our evolution from apes according to Darwin or our moulding from clay according to The Bible. I am reluctant to dive into the controversy betweent he two, and I think I have found a more interesting way to explore the question.

The world we live in at the moment is covered with all kinds of people, each individual originating from a different part of the globe and displaying different physical characteristics. I've always been interested in why we all look different as a result of where we come from on the earth.What is it about Nigeria that makes me have brown skin and curly hair, whilst my Japanese friend has white skin and straight hair. Is it something in the air or the soil?

What genes and Fossils tell us!

A very interesting article I stumbled across by national geographic gave a simplified and brief explanation of how humans today, who look like me and you, travelled from africa thousands of years ago to populate the rest of the earth, Asia, Europe, America, Australia and the rest. This migration which led to the population of the rest of the earth took thousands of years and is the reason why, although we are all the same we can look very different.

Geneticists and anthropologists agree that humans that look like us today arose from africa, around 200,000 years ago. The oldest human fossils have been found there and africans display the widest genetic diversity.

For more info from national geographic

http://ngm.nationalgeographic.com/ngm/0603/feature2/map.html

In my next post, I will be looking into what route humans took out of Africa, why we decided to leave and what happened to the other types of humans?

To be continued...