Evolution of modern humans
Where did we come from?
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.
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’
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.
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.
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.
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.
Admixture with extinct humans: what’s the evidence?
Are Neanderthals our cousins or ancestors?
Homo neanderthalis, or Neanderthals as they are more often known, are an extinct species of human that was widely distributed in ice-age Europe and Western Asia between 250,000 and 28,000 years ago. They were characterised as having a receding forehead and prominent brow ridges. In 1856 the first Neanderthal fossil was discovered in the Neander Valley near Düsseldorf in Germany. Since then, researchers have been striving to uncover the position of Homo neanderthalis in modern human evolution. Homo neanderthalis appeared in Europe about 250,000 years ago and spread into the Near East and Central Asia. They disappeared from the fossil record about 28,000 years ago.
Their disappearance has been put down to competition from modern humans, who expanded out of Africa at least 125,000 years ago (100,000-year-old remains of modern humans have been found in Israel), suggesting that there would have been a period of co-existence. Did the two species interbreed? Have Neanderthal genes therefore contributed to the modern human genome?
Initial studies of DNA from the mitochondria of Neanderthals showed that their mitochondrial DNA looks quite different to that of modern humans, suggesting that Homo neanderthalis and Homo sapiens did not interbreed.
Sequencing the Neanderthal genome
In 2010, scientists from Germany and the USA sequenced the DNA of an entire Neanderthal genome. They also identified another archaic human group called ‘Denisovan’, named after the Siberian cave in which the fossil finger, from which the DNA was obtained, was discovered. In 2013 they obtained a more refined Neanderthal genome sequence from a 50,000-year-old Neanderthal toe bone, found in the same cave in southern Siberia.
DNA can survive in bone long after an animal dies. Over time the DNA from various microbes that encounter the skeleton will also invade the bone. As a result, the DNA can be contaminated with microbe DNA. Scientists therefore have to ensure that they sequence only the Neanderthal genome and get rid of any DNA material left behind by these microbes or resulting from contamination by modern humans who handle these bones. As with the human genome sequence, the Denisovan and Neanderthal genome sequences were made available online for free. The genome sequence suggested that early modern non-African humans interbred with their now extinct ancient human cousins as they journeyed along coastlines and over mountains.
Analysis of the Neanderthal genome revealed that the toe bone came from a woman as it had two X chromosomes. Further analysis showed that each pair of chromosomes was similar in sequence. This suggests that her parents were closely related, perhaps an uncle and a niece. Inbreeding is generally bad for the genetic fitness of a species as it reduces the variation in a population making it more susceptible to disease and illness. This reduced genetic variation could explain why Neanderthals became extinct.
When comparing human genomes to the Neanderthal genome, human genomes resemble each other more than any of them resemble the Neanderthal genome. Some Neanderthal DNA is similar to DNA from people of European and Asian origin but these similarities are not seen in African DNA. This suggests that modern humans evolved in Africa and then expanded out into Asia and Europe, where Neanderthals lived. A degree of interbreeding between Neanderthals and early Homo sapiens then occurred in these areas. A study carried out in 2012 estimated that this interbreeding probably took place about 37,000-85,000 years ago and it is estimated that the proportion of Neanderthal-derived DNA in people outside Africa is 1.5-2.1 per cent.
From the past, to the future
Nowadays, many of us carry a small fraction of DNA from our archaic Neanderthal and Denisovan ancestors. This shared DNA could have shaped our individual susceptibility to modern-day diseases or adaptation to new environments and climates. Scientists have found nine Neanderthal genes in living humans known to be associated with susceptibility to conditions such as type 2 diabetes?, lupus and Crohn’s disease. It has also been shown that high-altitude adaptation in Tibetans may be a consequence of archaic Denisovan DNA sequence in a region of DNA associated with haemoglobin concentration at high altitudes. Additional research is being carried out to investigate these links further.