Our Distant Ancestors
How We Came to Be the Only Humans on Earth
St. Martin's Griffin 2013
About the Author: Chris Stringer is a Research Leader in Human Origins at the Natural History Museum in London, and a Fellow of the Royal Society. He is one of a small group of researchers who first proposed that H. sapiens evolved in Africa and migrated to the rest of the globe, which has become the leading theory of the origin of modern humans. He has authored numerous popular articles and books on human evolution, and is often interviewed when new and noteworthy discoveries find their way into the press. Prof. Stringer’s special interests and expertise concern Neanderthals and the first ancient human inhabitants of the British Isles.
While we may be able to recognize the humanity in the faces depicted for early hominids, we can easily distinguish them from our fellow modern humans. So how did these archaic humans evolve to become us? Since the first discovery of the remains of ancient humans, called Homo habilis, by the Leakeys in the 1960s at Olduvai Gorge, Africa has been recognized as the birthplace of humanity. About two million years ago H. Erectus, who evolved from H. habilis left Africa and spread throughout the Old World. What happened next is uncertain.
Until late in the 20th century the main theory was the Multiregional Hypothesis (‘MRH’), which proposes that modern humans evolved more-or-less independently in Europe and Asia from those first humans, who left Africa. An alternative is the Recent Out of Africa theory, (‘ROA’), which theorizes that H. sapiens evolved roughly 200,000 years ago (200 “kya”) in Africa from the descendants of the H. erectus who had remained there. Then, around 100 kya, some H. sapiens followed their antecessors across Asia and Europe, supplanting them wherever they met by one means or another, but not interbreeding with them.
ROA quickly grew in favor after a study of 147 people from around the globe indicated that their lineages could all be traced back to a single female ancestor, popularly referred to as “African Eve”. The findings strongly implied that modern humans from Africa were a new species that replaced rather than interbred with indigenous predecessors worldwide.a
Denisovans and interbreeding
Approximately two million years elapsed between the time H. erectus left Africa and H. sapiens followed. During this span, some African H. erectus evolved into H. heidelbergensis, named for the location in Germany where their remains were first discovered, and some of these also left Africa. The H. heidelbergensis who remained behind went on to become our species around 200 kya.
For the H. heidelbergensis who left, migrations and climate changes led to groups of them becoming isolated from one another more than 300 kya.b
One group migrated into western Asia and Europe, now known as Neanderthals.
Another group moved further eastward into Asia. Research excavations in this part of the world are more difficult than in Africa, and knowledge of this eastern group has come to light only recently in the discoveries at a cave in southern Siberia. Called Denisova after an 18th century hermit occupant, the cave holds forensic evidence of early habitation by both Neanderthals and H. sapiens, and a few small bones of a species new to us.
The cold temperatures in the caves preserved genetic material, which allowed an analysis of the DNA taken from the pinky bone of a girl found at Denisova. This child, who lived about 40 kya, was possibly from an unknown species of human. More surprisingly, her DNA matched about 4% to 6% of the DNA of the modern humans indigenous to Australia, Melanesia, and other areas of the western Pacific. This distribution suggests that these “Denisovans” had journeyed as far away as Southeast Asia, where they must have interbred with modern humans.c
Neanderthals and interbreeding
It has been known for decades that H. sapiens and H. neanderthalensis lived side-by-side after the moderns arrived, in caves around Mt. Carmel in present day Israel and other locations. It has long been speculated that the two groups exchanged more than pleasantries. Confirmation that moderns interbred with Neanderthals as well as Denisovans came from the Max Planck Institute team that discovered the unique Denisovan genetic signature.After sequencing the Neanderthal genome from remains found in Croatia, the team compared this with the genome of modern people from China, France, Africa, and New Guinead and found that all non-Africans today have as much as 4% of Neanderthal DNA in their genome and that the interbreeding happened soon after H. sapiens arrived in southwest Asia.e
This interbreeding may have helped our ancestors acclimatize in regions where they were newcomers but where Neanderthals had become well adapted. We inherited thicker skin for adapting to cold from Neanderthals, and there are indications as well that interbreeding resulted in changes to our immune system, although perhaps not necessarily for the better.
One species or many?
Dmanisi has been an active excavation site for decades. Remains found at Dmanisi, in the Republic of Georgia in 1991 dated as 1.8 million years old are the oldest hominid remains found outside of Africa (these remains are classified as H. erectus – more will be said below about this). The more recent discovery there in 2005 of a complete skull found together with the remains of four other individuals presented an intriguing challenge. This skull has a braincase that is remarkably small, and notably smaller than the remains of the other four hominids, which all varied from one another to a remarkable degree and yet were individuals who lived at the same location around the same time.
Finding the Dmanisi skull with similar remains prompted researchers to compare the variation in these remains with the variation found among modern humans. They concluded that the Dmanisi variations were no greater than those seen in modern people, and proposed that they may all be variations of H. erectus.
So, was H. erectus the only precursor to modern humans and all other findings just variants of them, as multi-regionalists contend? Stringer believes that Neanderthals and Erectus are recognizably separate from moderns and from one another – with their prominent brow ridges, receding chins and less rounded skulls. Moreover, Neanderthal DNA differs from modern DNA by at least twice the largest difference found between any modern people.f
Homo erectus was alive in East Africa at least 1.8 million years ago. An adult was roughly 5 feet tall, with a brain about half the size of a modern human but considerably larger than H. habilis, the first of the Homo genus. Erectus made basic stone tools of the same unchanging design for thousands of years, but despite these limitations they spread as far as China and Java. It isn’t known why H. erectus eventually became extinct – they were still in China until about 300,000 years ago and possibly quite a bit more recently – existing for well over a million years.In Stringer’s opinion, there is one intermediate species between H. erectus and H. sapiens, named H. heidelbergensis, also the progenitor of Neanderthals and Denisovans. H. heidelbergensis lived from perhaps one million years ago until about 200 kya, and possessed a brain almost as large as moderns. Our lineage appears then as shown:g
H. heidelbergensis were about the same size as Neanderthals, about our height but more robust. They buried their dead, presumably the first humans to do so. Some paleoanthropologists believe H. heidelbergensis showed evidence of the first beginnings of language. And they, too, left Africa at times to wander Asia and Europe, where some of them developed into Neanderthals, Denisovans, and perhaps others still to be discovered – and ourselves.