Evolution of modern humans

The origin of modern humans has probably been the most debated issue in evolutionary biology over the last few decades. 

Where did we come from?

“The exact origin of modern humans has long been a topic of debate.

Our evolutionary history is written into our genome?. The human genome looks the way it does because of all the genetic changes that have affected our ancestors. The exact origin of modern humans has long been a topic of debate.

KEY FACTModern humans originated in Africa within the past 200,000 years and evolved from their most likely recent common ancestor, Homo erectus.

Modern humans (Homo sapiens), the species? that we are, means ‘wise man’ in Latin. Our species is the only surviving species of the genus Homo but where we came from has been a topic of much debate. Modern humans originated in Africa within the past 200,000 years and evolved from their most likely recent common ancestor, Homo erectus, which means ‘upright man’ in Latin. Homo erectus is an extinct species of human that lived between 1.9 million and 135,000 years ago.

Historically, two key models have been put forward to explain the evolution? of Homo sapiens. These are the ‘out of Africa’ model and the ‘multi-regional’ model. The ‘out of Africa’ model is currently the most widely accepted model. It proposes that Homo sapiens evolved in Africa before migrating across the world.

On the other hand, the ‘multi-regional’ model proposes that the evolution of Homo sapiens took place in a number of places over a long period of time. The intermingling of the various populations eventually led to the single Homo sapiens species we see today.

“Current genomic evidence supports a single ‘out-of Africa’ migration of modern humans.

This is still very much an area of active research, however, current genomic evidence supports a single ‘out-of Africa’ migration of modern humans rather than the ‘multi-regional’ model. Although, studies of the genomes? of the extinct hominids Neanderthals and Denisovans suggest that there was some mixing of genomes (1-3 per cent) with humans in Europe and Asia.  This interbreeding between two previously separated populations is called ‘admixture’ and results in a mixing of genes? between the populations. 

‘Out of Africa’: what’s the evidence?

‘Mitochondrial Eve’

KEY FACTThere is more genetic diversity in Africa compared with the rest of the world put together.

Genetic studies tend to support the ‘out of Africa’ model. The highest levels of genetic variation? in humans are found in Africa. In fact there is more genetic diversity in Africa compared with the rest of the world put together. In addition, the origin of modern DNA? in the mitochondria (the ‘powerhouses’ of our cells) has been tracked back to just one African woman who lived between 50,000 and 500,000 years ago – 'Mitochondrial Eve'.

Our genomes are a combination of DNA from both our mother and father. However, mitochondrial DNA (mtDNA) comes solely from our mother. This is because the female egg contains large amounts of mitochondrial DNA, whereas the male sperm contains just a tiny amount. The sperm use their small amount of mitochondria to power their race to their egg before fertilisation. Once a sperm merges with an egg, all the sperm mitochondria are destroyed.

KEY FACTYour mitochondrial DNA is almost exactly the same as your mother’s and her mother’s.

As a result, mitochondrial DNA is described as being matrilineal (only the mother’s side survives from generation to generation). So, your mitochondrial DNA is almost exactly the same as your mother’s and her mother’s. Mitochondrial DNA has been extensively used by evolutionary biologists, as it is easier to extract than DNA found in the nucleus? and there are many copies to work with.

However, Mitochondrial Eve wasn’t the first or only woman on Earth at that time. She was simply the point from which all modern generations of human appear to have grown. Evolutionary biologists think the most likely reason for this is that an evolutionary ‘bottleneck’ occurred during the time Eve was alive. This is when the majority of a species suddenly dies out, perhaps due to a sudden catastrophe, bringing it to the brink of extinction. If Mitochondrial Eve was one of the few women to survive then this could explain why her ‘matrilineal’ mitochondrial DNA ended up being passed along so many generations.

Similarly, DNA from the Y chromosome? is only passed on from fathers to sons and a evolutionary tree relating all present day male individuals also supports the ‘out of Africa’ model.

Mapping skulls

Further evidence for the ‘out of Africa’ model can be found in the size of human skulls. After studying the genetics and skull measurements of 53 human populations from around the world, scientists found that as you move further away from Africa, populations are less varied in their genetic makeup. This may be because human populations became smaller as they spread out from their original settlements in Africa and so genetic diversity within these populations was less. As a result the scientists stated that modern humans could not have emerged in different places, but instead had to have come from one region, Africa.

KEY FACTThe oldest known remains of anatomically modern humans are the Omo I and Omo II skulls.

The oldest known remains of anatomically modern humans are the Omo I and Omo II skulls. These were found in 1967 in Omo National Park in south-western Ethiopia. The skulls have been dated to 195,000 years ago, highlighting how humans have evolved relatively recently.

Moving out of Africa

Evidence shows that the first wave of humans to move out of Africa did not have too much success on their travels. At times it appears they were on the brink of extinction, dwindling to as few as 10,000.

The eruption of a super volcano, Mount Toba, in Sumatra 70,000 years ago may have led to a 'nuclear winter', followed by a 1,000-year ice age. This sort of event would have put immense pressure on humans. It may be that humans were only able to survive these extreme conditions through cooperating with each other. This may have led to the formation of close family groups or tribes and the development of some of the modern human behaviours we are familiar with today, such as cooperation.

“Genetically, the six billion people of today’s world vary very little from the Homo sapiens that ventured out of Africa.

Between 80,000 and 50,000 years ago another wave of humans migrated out of Africa. These humans are likely to have been ‘modern’ in terms of their appearance and behaviour. Due to their newly cooperative behaviour they were more successful at surviving and covered the whole world in a relatively short period of time. As they migrated they would have encountered earlier, primitive humans, eventually replacing them. Genetically, the six billion people of today’s world vary very little from these earlier Homo sapiens that ventured out of Africa