COMMENT NATURAL HISTORY Edward EARTH SCIENCE How rocks and MUSIC Philip Glass on Einstein EMPLOYMENT The skills gained Lear’s forgotten work life evolved together on our and the unpredictability of in PhD training make it on ornithology p.36 planet p.39 opera composition p.40 worth the money p.41 ILLUSTRATION BY CHRISTIAN DARKIN CHRISTIAN BY ILLUSTRATION What makes a modern human We probably all carry genes from archaic species such as Neanderthals. Chris Stringer explains why the DNA we have in common is more important than any differences. n many ways, what makes a modern we were trying to set up strict criteria, based non-modern (or, in palaeontological human is obvious. Compared with our on cranial measurements, to test whether terms, archaic). What I did not foresee evolutionary forebears, Homo sapiens is controversial fossils from Omo Kibish in was that some researchers who were not Icharacterized by a lightly built skeleton and Ethiopia were within the range of human impressed with our test would reverse it, several novel skull features. But attempts to skeletal variation today — anatomically applying it back onto the skeletal range of distinguish the traits of modern humans modern humans. all modern humans to claim that our diag- from those of our ancestors can be fraught Our results suggested that one skull nosis wrongly excluded some skulls of with problems. was modern, whereas the other was recent populations from being modern2. Decades ago, a colleague and I got into This, they suggested, implied that some difficulties over an attempt to define (or, as PEOPLING THE PLANET people today were more ‘modern’ than oth- I prefer, diagnose) modern humans using Interactive map of migrations: ers. Although I disputed what I considered the skeletal morphology that is preserved in go.nature.com/nmz8gd to be a misuse of our test, I had to recognize fossils1. Our attempt was well-intentioned: the dangers inherent in this prescriptive 3 MAY 2012 | VOL 485 | NATURE | 33 © 2012 Macmillan Publishers Limited. All rights reserved COMMENT approach to modern human variation. originated through different mechanisms that the Neanderthals represented a lineage Today, scientists are facing a similar and over different time scales3. The first set and species that was separate from all recent situation. In 2010, DNA evidence showed includes features shared by all living humans humans. However, increasingly complete that after modern humans left Africa about — such as a high and rounded skull, small genomic reconstructions from Neanderthal 60,000 years ago, they bred for a short period brow ridge and a bony chin on the lower fossils4 and from a newly characterized group of time with archaic humans — and, as a jaw3 — that make it clear a skull belongs to called Denisovans — so far known only from result, some populations today have more H. sapiens, and not an ancient form. These a single cave in southern Siberia5 — have archaic genes than others3. These genes traits had evolved in Africa by 100,000 years shown these hominins to be related popula- might be expressed in the phenotype, and ago, and were exported from there in an exo- tions that descended from the earlier species may require a rethink about how and when dus that began about 60,000 years ago. Homo heidelbergensis, which also gave rise to regional variation developed in H. sapiens. The second set of traits belonging to H. sapiens (see ‘A winding path’). Those with alternative agendas may also modern humans Comparative genomic studies have try to use these new data to rank modern “It is not yet include the regional revealed elements of DNA that are unique human populations in terms of supposedly clear whether or ‘racial’ features that to each of the three groups (recent humans, different degrees of modernity. Already I’m the archaic differentiate human Neanderthals and Denisovans), some of reading blogs that speculate about whether DNA many of populations, such as which could be expressed in the phenotype, some groups are less ‘modern’ than others, facial shape, form of and that may be related to such things as the and I fear that such discussions endanger the us carry is tied the eyelids, type of physiology of the brain, skin, skeleton and considerable progress promised by palaeo- to any visible hair, skin pigmenta- even sperm4,5. But the biggest surprise for genetic research. traits.” tion and physique. many researchers was the evidence from So to highlight these issues and steer the I have argued that whole genome scans that modern humans debate in a positive direction, I would like these regional features were added as peo- living outside Africa each carry about 2.5% of to assert that the term modern humans, by ple spread in small numbers after the shared their DNA from Neanderthals; furthermore, definition, equally describes all humans liv- modern human template had evolved in people living today in Australia and New ing today. Some of us may have more DNA Africa. The processes involved were natu- Guinea (Australasians) carry about 5% of from archaic populations than others, but ral selection (Charles Darwin’s favoured Denisovan DNA4,5 (see ‘Patchwork planet’). the great majority of our genes, morphol- evolutionary mechanism in his 1859 On the The most likely explanation is interbreed- ogy and behaviour derives from our com- Origin of Species), different sexual or cul- ing, such as when modern humans emerged mon African heritage. And what unites us tural preferences (emphasized in Darwin’s from Africa into Eurasia about 60,000 years should take precedence over that which 1871 volume The Descent of Man), a founder ago and met Neanderthals, and when the distinguishes us from each other. effect as remote regions were colonized by ancestors of Australasians met some Den- small pioneer groups, and drift, as newly isovans. Scientists are continuing to study OUT OF AFRICA separated populations diverged through these archaic genomes in detail, and are find- For the past 25 years, there has been a fierce random processes. ing clear variation within and between non- debate about whether our species evolved That dispersal event potentially brought African people in the amount and kind of from distinct ancestors who were spread early modern humans into the realms of archaic DNA that survives in their genomes across the Old World (multiregional evo- other hominin populations living outside today. In short, people living outside Africa lution) or whether, as I have advocated, we Africa, including the Neanderthals in western carry different quantities, and distinct evolved recently in only one area — Africa Eurasia, and Homo floresiensis and the ancient remnants, of archaic DNA from those inter- (recent African origin). I have argued that species Homo erectus in Indonesia. The first breeding events. Some African populations the physical traits of modern humans can be DNA to be successfully recovered from Nean- may also contain genes from ancient groups divided into two sets of features, which have derthal fossils seemed to support the view as a result of interbreeding, as shown by recent DNA evidence6 and 15,000-year-old A WINDING PATH fossils from Central and West Africa, which H. sapiens spread from Africa to After early modern humans left Africa around 60,000 years ago (top have some ancestral traits such as a longer, western Asia and then to Europe and 3 right), they spread across the globe and interbred with other southern Asia, eventually reaching lower braincase . descendants of Homo heidelbergensis. Australasia and the Americas. This evidence of interbreeding between 0 groups that palaeontologists call separate Homo sapiens species raises two crucial questions. First, Denisovans Neanderthals Homo oresiensis given that most of us learned at school that Homo erectus 0.4 species don’t interbreed, should we change the definition we use for a species? Or Homo heidelbergensis should we remove the taxonomic separa- 0.8 tions erected purely from the morphology of Homo antecessor fossils, and sink H. heidelbergensis, Neander- thals and Denisovans into H. sapiens? And H. heidelbergensis originated from second, how does the evidence of interbreed- 1.2 Million years ago H. erectus in an ing affect our concept of modern humans, unknown location and dispersed across when the genomes of people today appar- Homo erectus Africa, southern Asia ently contain differing levels of archaic genes? and southern Europe. 1.6 LAID TO REST H. oresiensis originated H. erectus spread to western Asia, then in an unknown location east Asia and Indonesia. Its presence In my view, the evidence that H. sapiens and reached remote in Europe is uncertain, but it gave rise interbred with archaic humans does not yet 2.0 parts of Indonesia. to H. antecessor, found in Spain. require a merging of these close relatives Wavy branch edges suggest presumed uctuations in population. into a single expanded concept of H. sapiens. 34 | NATURE | VOL 485 | 3 MAY 2012 © 2012 Macmillan Publishers Limited. All rights reserved COMMENT PATCHWORK PLANET Most people’s genomes contain remnants of archaic DNA from ancient interbreeding3–6. 2% 2.5% 2.5% 5% Genes* African Unknown archaic African source 98% 97.5% 92.5% Neanderthal Denisovan *Figures are approximate, and for Africa, based on limited data6. Sub-Saharan Africa Eurasia and Americas Australia and New Guinea Doing so would produce a species that had to continue the progress recently made in a range of morphological variation sev- studying the origins of modern human vari- eral times that found in humans today, or ation, they will need to think long and hard in other existing primate species. These about their aims, and the lexicon they use. human lineages were distinct enough to One thing should be reiterated: all living build up well-differentiated genotypes and humans are members of the extant species phenotypes (although we know little of the H.