The human family tree is branching rapidly. Since 2000 alone, at least four new extinct human species have been discovered. Putative others awaiting certification with Latin binomials look set to add further blossom. The tree it seems is becoming more of a bush, worthy of the savannah landscape and scene of our emergence. So, what is it that makes our own species Homo sapiens distinct? How do we differ from the likes of the Neanderthals? Where and when did the classic “modern human” looks appear and our core cultures originate?
The human family tree including all of its current known members. Annotated on are two recent additions - Australopithecus sediba and the genetically-described Denisovan lineage known from a tooth and finger bone discovered in Siberia.
Human origins are notoriously “incomplete and patchy” says Chris Stringer, Professor of Palaeoanthropology at the Natural History Museum, London. The ‘human phase’ in evolution is marked by defining trends in traits. These include an increase in brain size, dietary range and the complexity of our technology; all the way through stone, bronze and iron tools to computers and nanoscale inventions.
Stringer is a major proponent of the Out of Africa 2 theory - the model used to describe our human ancestors dispersal from the African heartland some 80,000 years ago, from which all our roots can be traced. The accepted model dictates that Homo sapiens completely replaced the likes of Homo erectus and the Neanderthals throughout Eurasia during a single mass exodus.
In physical appearances, modern humans differ noticeably in portrait compared to extinct species like Homo heidelbergensis and Homo neanderthalensis (Neanderthals, pictured below). Our faces are tucked under our braincase unlike the primitive, protrusive state of archaic species. We are overall leaner and less robust than our conical-ribbed cousins the Neanderthals even if our brain sizes were on a par, if not inferior.
A selection of Neanderthal skullcaps - note the bulging brow ridges, facial prognathism and low cranial vault unlike the high, domed skull of H. sapiens.
Our behaviour also distinguishes us from the growing crowd of extinct relatives. We live in civilised dwellings and possess the exquisite abilities to paint a picture or intricately thread a needle in order to sew. Language and writing have propelled our species successfully through the cultural revolutions, enabling us to better communicate and record.
All these favourable characteristics must have benefited early Homo sapiens, helping us to conquer the Earth. Interestingly though, due to the many sub-populations in Africa, not all aspects of modern human anatomy and culture were instantaneously knotted. Instead, the assembly of all the modern traits into our H. sapiens ancestors is seen as a kind of genetic fission, dubbed “piecemeal coalescence”. To add to the complications, the African climate was somewhat variable in the early years of our young species which limited population sizes to a few hundred at most. Mitochondrial Eve and her companions it seems were in no way destined for world dominance.
The sites bearing human fossils with the earliest recognisable modern features are exotically named places such as Omo Kibish and Herto, from Ethiopia in East Africa. In age the fossils are 195-160 thousand years old which is in accordance with the predicted genetic age for the origins of H. sapiens. Even older is a skull from Florisbad, South Africa that dates to 260 thousand years ago. The Florisbad skull possesses an array of features intermediate between H. sapiens and our supposed direct Africa ancestor - H. heidelbergensis. If one thing then is clear, it’s that the appearance of our species in the fossil record was not.
Human remains from Israel dating to ca 125,000 years ago further distort the already fuzzy picture of early H. sapiens. These younger fossils constitute the earliest defining evidence of our species roaming outside of Africa. The enigmatic skullcaps aren’t typically modern human though and instead exhibit some classic Neanderthal features such as strong brow ridges. The common consensus now is that the Israel fossils represent an early wave of H. sapiens from Africa, pre-dating the archetypal migration categorised as Out of Africa 2 (OOA2).
In light of shocking new evidence, Out of Africa 2 has been forced to revise. It transpires that DNA from the Neanderthals and the even more mysterious Denisovan group is found in the genetic code of us - modern humans. Only interbreeding can explain such an occurrence. New adaptations of OOA2 are being labelled “leaky replacement” to account for the admixture caused by our ancestors’ apparent promiscuity.
The enigmatic cranium of 'The Hobbit' found on the island of Flores, Indonesia - showing signs of asymmetry?
There is no doubt then that the unauthorised biography of our species cannot yet be written confidently. With constant developments always changing the picture and the branches of our family tree beginning to intertwine, many surprises still lay in wait. Unsolved cases remain, for example the nature of the curious, diminutive hobbits of Flores (left); were they diseased H. sapiens, pygmy individuals or a distinct species altogether? Only time will tell.
Our attempts to classify all that we observe are futile. Debate over the implications of new discoveries and slight contentions will forever ensue, as sure as the eminence of man from within the great apes. If one thing can be agreed upon it’s that human evolution is truly captivating.
This article is based on a seminar delivered to the University of Sheffield’s Archaeology Department on (23/03/2011) by Prof. Chris Stringer . The seminar was entitled “Physical and Behavioural Origins of Modern Humans“.