Over the last few decades, the origin of modern humans has likely been the most contentious issue in evolutionary biology.
What happened to us?
Is our evolutionary history encoded in our genome? The human genome is the result of all the genetic changes that have occurred in our ancestors. The precise origin of modern humans has long been debated.
In Latin, modern humans (Homo sapiens) are referred to as “wise men.” Ours is the only surviving species of the genus Homo, but where we came from has long been debated. Within the last 200,000 years, modern humans evolved from their most likely common ancestor, Homo erectus, which means “upright man” in Latin. Homo erectus was a human species that existed between 1.9 million and 135,000 years ago.
Two major models have been proposed in the past to explain the evolution of Homo sapiens. These are the “out of Africa” and “multi-regional” models.
The’multi-regional’ model, on the other hand, proposes that Homo sapiens evolved in a number of locations over a long period of time. The interbreeding of the various populations resulted in the current single Homo sapiens species.
Although this is still an active area of study, current genomic evidence supports a single ‘out-of-Africa’ migration of modern humans rather than the’multi-regional’ model. Despite this, studies of the genomes of the extinct hominids Neanderthals and Denisovans suggest that there was some genome mixing (1-3%) with humans in Europe and Asia. Interbreeding between two previously separated populations is known as admixture, and it results in gene mixing between the populations.
What is the evidence for ‘Out of Africa’?
Genetic studies tend to back up the ‘out of Africa’ theory. Africa has the highest levels of genetic variation in humans. In fact, Africa has greater genetic diversity than the rest of the world combined. Furthermore, the origin of modern DNA in the mitochondria (our cells’ ‘powerhouses’) has been traced back to a single African woman who lived between 50,000 and 500,000 years ago – ‘Mitochondrial Eve.’
Our genomes are made up of DNA from both our mothers and fathers. Our mother, on the other hand, is the sole source of mitochondrial DNA (mtDNA). This is due to the fact that the female egg contains a large amount of mitochondrial DNA, whereas male sperm contains only a trace amount. Before fertilization, the sperm use their small amount of mitochondria to power their race to the egg. When a sperm and an egg combine, all of the sperm mitochondria are destroyed.
What is the evidence for admixture with extinct humans?
Are Neanderthals our ancestors or cousins?
Homo neanderthalis, also known as Neanderthals, is an extinct species of human that lived in ice-age Europe and Western Asia between 250,000 and 28,000 years ago. They were distinguished by a receding forehead and prominent brow ridges. The first Neanderthal fossil was discovered in the Neander Valley, near Düsseldorf, Germany, in 1856. Since then, scientists have worked to determine the role of Homo neanderthalis in modern human evolution. Around 250,000 years ago, Homo neanderthalis appeared in Europe and spread to the Near East and Central Asia. They vanished from the fossil record approximately 28,000 years ago.
Their extinction has been attributed to competition from modern humans, who migrated out of Africa at least 125,000 years ago (100,000-year-old modern human remains have been discovered in Israel), implying that there would have been a period of coexistence. Did the two species mate? So, did Neanderthal genes contribute to the modern human genome?
Initial studies of DNA from Neanderthal mitochondria revealed that their mitochondrial DNA differed significantly from that of modern humans, implying that Homo neanderthalis and Homo sapiens did not interbreed.
From the past to the present 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.