My wife has always worried about me going bald, because I have a bump on the back of my skull that if exposed would make me look like a Neanderthal. Common knowledge still maintains that Neanderthals were rather stupid when compared to Homo sapiens, the species we all belong to today. There is, or was, a “gap” between them and us, rather like the gap that Thomas Suddendorf writes about when comparing the mental abilities of modern humans and the great apes. When I used to lecture on human evolution, my hand would involuntarily stroke my head to feel my “occipital bun” just as I mentioned those words to describe a key anatomical difference between Homo sapiens and Homo neanderthalensis. Should I just offer myself as a live demonstration?
My wife need worry no more and my students have had a lucky escape. By finding the lost genome, the Swedish biologist Svante Pääbo has discovered that we are all part Neanderthal—except those with an entirely African heritage. So I now feel like shaving my head to celebrate the interspecies engagement of 50,000 years ago, whether or not it is the ultimate cause for the oddity of my skull and my occasional acts of stupidity.
Archaeologists and physical anthropologists have long debated the evolutionary relationship between modern humans and Neanderthals, relying on the similarities and differences between their designs of stone artifacts and the shapes of their bones, with little real understanding of how these might have arisen. Interminable academic arguments have been swept away by the revolution in studies of ancient DNA, led by Pääbo (now at the Max Planck Institute in Leipzig) and brilliantly recounted in his new book, Neanderthal Man: In Search of Lost Genomes.
Pääbo has provided us with a fabulous account of three decades of research into ancient DNA, culminating in 2010 with the publication of the Neanderthal genome. It’s a story seen through his eyes. He describes how he began with secretive attempts to explore whether DNA would survive in an artificially mummified calf’s liver, and how he led a multimillion-dollar global research project to ascertain the genomes of ancient organisms.
Pääbo’s book has to be compared to The Double Helix (1968), James Watson’s brilliant but controversial account of how the structure of DNA was discovered. When taken together they provide an insight into how biomolecular science has both changed and remained much the same during the last half-century. Both are strong personal accounts of scientific discovery, exposing how science is driven as much by passion, ambition, and competition as by rational thought and the sharing of knowledge. In both books the reader is gripped by life stories of far greater interest than those in many novels before being plunged into passages of near-unintelligible science (despite much simplification) that are nevertheless strangely enthralling.
Pääbo comes across as a generous scientist compared to the distinctly unlikable Watson of The Double Helix. Unlike Watson’s frequent disparaging of others, Pääbo criticizes chiefly those who make “outlandish” and “ludicrous” claims regarding the extraction of DNA from fossils that are millions of years old, obstructionist museum curators, and occasionally himself.
The pressure to publish was present in both worlds. For Pääbo it was especially intense, and partly self-inflicted. One of the reasons he became disillusioned with his early interest, during the 1980s, in Egyptology and archaeology was the “glacial speed” with which the journals “concerned with ancient things” were moving. By the time his article on the extraction of ancient DNA from an Egyptian mummy appeared in the Journal of Archaeological Science in 1985, it had already been overtaken by events, primarily dramatic advances in scientific techniques.
So Pääbo chose to work in a fast-moving field in which journals and prospective authors have to move at an equivalent speed. His decisions about which journals to publish in, when to submit, and what to include in any single paper, along with his reactions to referees’ comments and his responses to the publications of others—sometimes being inspired, at other times distraught—pervade the text as they do the life of any scientist in a fast-moving field today. (Archaeology remains relatively sluggish.) The magnum opus publication of the Neanderthal genome in Science in 2010 had fifty authors. It took up a mere twelve pages in the journal, supported by 174 pages of technical detail in the online supplementary material. An utterly fascinating chapter is devoted to the preparation and reception of the manuscript.
If the pressures, pace, and technical demands of publication have intensified since Watson’s time, the peculiar blend of cooperation and competition between scientists appears much the same. For Watson the competition-cooperation was just down the road at King’s College London and in the Pasadena laboratory of Linus Pauling; for Pääbo and his team, based in the Max Planck Institute, it was Eddy Rubin at Berkeley. He had been a collaborator, but adopted a different method for the sequencing of nuclear DNA than Pääbo, preferring to use the method of cloning bacteria rather than adopting the new polymerase chain reaction (PCR) method. Rubin suffered the consequence of making the wrong choice, becoming little more than a footnote in the story of how ancient DNA was reconstituted (at least as recounted by Pääbo—Rubin might see it differently, as did many of those who read Watson’s book).
The impact of new technology was critical to the successes of both Watson and Pääbo—although far more evident in the latter. Pääbo describes his twenty-five years in molecular biology as a continuous technological revolution. He learned that “unless a person was very, very smart, breakthroughs were best sought when coupled to big improvements in technologies.” Watson and his collaborator Francis Crick are often idealized (notably by Watson himself) as playing with bits of cardboard when building their double helix model for DNA. They could only do so, however, because of the advances in X-ray diffraction that had enabled Rosalind Franklin to create images of DNA that revealed its helical structure. A sneaky look at a single image of Franklin’s by Watson was sufficient.
Pääbo’s dependency on technology was quite different. When hearing about the PCR method at a conference in 1986 he immediately recognized a technical breakthrough for studying ancient DNA. Two decades later, when ready to sequence the three billion nucleotides of the Neanderthal genome, Pääbo required the support of a biotechnology company called 454 Life Sciences, along with several million pounds to pay them. Coincidentally, another project of 454 Life Sciences was “Project Jim,” sequencing the whole genome of James Watson.
While The Double Helix covers just three years (1951–1953) and a single discovery (albeit one on which so much else is based), Pääbo’s search for lost genomes spans three decades and takes the reader through a linked series of discoveries: from the first extraction of Neanderthal DNA to the mapping of the whole Neanderthal genome, the discovery of interbreeding between Neanderthals and Homo sapiens, and the identification of a brand-new human species from ancient DNA that had been extracted from a finger bone found in Denisova Cave in Siberia. Along the way we learn about the extraction of ancient DNA from Egyptian mummies, Oetzi the Bronze Age Ice Man, mammoths, and cave bears. The technical breakthroughs are critical to the story, none more so than those that enabled the shift from studying mitochondrial DNA—i.e., the DNA that is stored in hundreds of copies in a cell’s tiny structures outside its nucleus—to nuclear DNA, two copies of which are stored in the cell’s nucleus. The mitochondrial DNA is inherited from the female line alone and hence provides what Pääbo calls a “blurry, one-eyed view of human history.”
A constant theme of Pääbo’s book is the battle to detect and avoid contamination of ancient DNA by that of modern humans. Such contamination might derive from the archaeologists who innocently handled the bones during excavation or the scientists whose DNA is unavoidably carried on the dust floating through their laboratories. Others should have known better: Pääbo describes his horror at watching a curator at the Natural History Museum in London lick an ancient bone to detect whether it had once been chemically treated, thus coating it with his own DNA. This is one of several moments of human folly interspersed through Pääbo’s account of high-tech science. Quite different is Pääbo’s savoring of the smell of burnt bone when a Neanderthal arm bone was being cut to provide a sample for analysis—the smell suggested that collagen within the bone had survived and hence DNA would be found.
Finding such bones was another challenge for Pääbo, and led to battles with obstructionist museum curators and then negotiations with the Croatian Academy of Sciences and Arts to access the Neanderthal remains from Vindija Cave in the north of the country. Luckily he succeeded, enabling him to find what he describes as a “magic bone,” one that had almost 3 percent of its Neanderthal nuclear DNA preserved, in contrast to the usual frequency of between 0.06 and 0.2 percent. This crushed fragment of bone, five centimeters long, enabled the sequencing of the Neanderthal genome and has changed our understanding of human history. It had sat unremarked upon in a museum basement for several decades.
Throughout this story of ancient DNA we also follow the human story of Pääbo as he drives the science forward, which was by no means a smooth ride. He is an emotional man: on one page we find him overexcited, on another insecure; he is periodically frustrated, irritated, and feels like screaming; he can be very happy and then deeply disappointed. He admits to making a fool of himself, suffering from an exaggerated desire to be liked, bouts of sarcasm (swiftly regretted), and being very sentimental. His emotions are critical to his scientific success and when we read about his brief infatuations with heterosexual men and his pleasure at strolling along a beach naked with his new wife, these passages do not seem inappropriate. His success was not only based on his individual efforts but on building a team of scientists at the Max Planck Institute. He had to manage this team so that each member could pursue his or her own goals while contributing to the big project—budding scientists are, he writes, largely driven by self-interest. When Pääbo writes about how he felt a love for each and every person around the table, one believes him and appreciates that this was crucial to his, and their, success.
What did that success amount to? Pääbo and his team were able to determine that the modern human and Neanderthal lineages had split between 270,000 and 440,000 years ago, a conclusion that confirmed existing views. Far more surprisingly, however, they concluded that people outside of Africa, whether Europeans or Asians, have up to 5 percent of their DNA derived from Neanderthals. The most likely scenario is that a group of modern humans left Africa sometime around 50,000 years ago, interbred with Neanderthals in the Middle East, and then went on to populate the world, taking the Neanderthal DNA with them. That DNA would have been diluted with every generation of humans but its persistence into the present day suggests that some of the Neanderthal gene variants have provided modern humans in Europe and Asia with enhanced adaptive capabilities over their African Homo sapiens forebears.
By drawing on the work of the English biologist Peter Parham, Pääbo was able to offer one possible example. Parham, we learn, is one of the world’s leading experts on the “major histo-compatability complex” (MHC), a particularly complicated genetic system in the human genome associated with fighting infection. He found that one particular MHC gene variant is common in today’s Europeans and Asians but unknown in Africans. Pääbo found this gene variant in the Neanderthal genome. As such, it must have been inherited from the Neanderthals by those humans who first ventured into the Middle East and then undergone positive selection to reach its frequency in modern-day non-Africans. Parham suggests that this particular gene variant provided an advantage in fighting off diseases that were local to Europe and Asia.
The nature and extent of the differences between Neanderthals and Homo sapiens—the gap—remain as vigorously debated as ever. Pääbo and his team identified the molecular gap between them. There are seventy-eight amino-acid-altering nucleotide positions—“meaningful mutations”—in which all humans today are similar to one another and different from Neanderthals and the apes. Pääbo suspects that this number will reach two hundred as the sets of data are refined but even that will be only a tiny proportion of the complete human genome.
These amino acid differences arose from mutations after the lineage divide that led to the emergence of both Neanderthals and Homo sapiens from a common ancestor. They influence the proteins that build tissues and the mind. Considering that there are more than 20,000 proteins encoded by the genome, a mere seventy-eight (or even two hundred) mutations seem a tiny genetic gap—but the behavioral consequences might be vast. Regrettably, we have little idea of any such consequences. Pääbo confesses all: “The dirty little secret of genomics is that we know next to nothing about how a genome translates into the particularities of a living and breathing individual.” As such, despite all we know about ancient DNA, arguments about the relative cognitive capabilities of modern humans and Neanderthals remain reliant on interpretations of the archaeological data. My own view is that these capabilities were quite different.
Thomas Suddendorf would probably consider me a “killjoy,” his term for those who are reluctant to ascribe complex humanlike abilities to other animals. He contrasts the killjoys with the “romantics,” those who are more prepared to see greater continuities (even the most deadly of killjoys must accept some continuities). Although Suddendorf often adopts the middle ground, his use of these terms is telling: Is it not rather romantic to think, as he often does, that humans are quite different from all other humans and rather a killjoy attitude to think that we are just part of the mix?
The Gap: The Science of What Separates Us from Other Animals is about a challenge quite different from Pääbo’s quest to find out what separates us from the Neanderthals. Suddendorf’s evidence comes from behavioral observations, primarily of the great apes and most significantly of chimpanzees, the living species to whom we are most closely related. From these observations he seeks to infer the similarities and differences in their mental capabilities from those of modern humans. Of course, differences are undeniable—otherwise a chimpanzee might be writing this review—but their nature and extent have been long debated with no satisfactory resolution. It appears that psychologists can display equal degrees of rigor in their experimental designs and data analysis and yet reach quite different interpretations about the size of the gap; psychologists can be either killjoys or romantics.
In this regard, Suddendorf’s science of comparative psychology is quite different from the molecular biology described by Pääbo and Watson. Molecular biology has answers that are more evidently right or wrong and hence have a far greater degree of measurable progress. Observations made by psychologists in the 1950s and earlier appear just as relevant to Suddendorf’s evaluations as those from recent times, while many of the questions being asked remain those raised by Darwin. I don’t suppose Pääbo looks to publications more than twelve months old in his fast-moving field. In other respects there are similarities. Suddendorf’s science is certainly driven by competition, such as that described between Daniel Povinelli’s team at the University of Louisiana (killjoys) and Michael Tomasello’s team at the Max Planck Institute in Leipzig (romantics, coincidently based at the same institute as Pääbo).
While there is an element of personal journey in his account, Suddendorf’s book is a work of synthesis. It draws on the research of many psychologists, including his own important contributions, making what he describes as a “prudent and cautious” analysis. It covers an extensive number of observations made in the wild and in laboratory experiments, describing the various interpretations for the underlying mentality of apes and humans.
The book is structured around six fields of inquiry: language, mental time travel (through memory and anticipation), mind reading (through empathy and imagination), intelligence and problem-solving, cultural transmission, and morality. Each has a chapter in which Suddendorf initially summarizes the principal characteristics of human mentality, then provides comparative evidence from the great apes (and occasionally other animals), and finally makes a comparison. The boundaries between the six subjects are extremely blurred—one can hardly write about language without referring to mind reading or about cultural transmission without language.
It is not surprising, therefore, that Suddendorf concludes that all six fields share two features that set humans apart from our closest relatives: “our open-ended ability to imagine and reflect on different situations” and “our deep-seated drive to link our scenario-building minds together.” Unfortunately, I remained unclear whether he was suggesting that these features provide the underlying foundations for language, intelligence, and so forth, presumably requiring some biological basis in the brain, or whether they represent emergent properties from the interactions between his six fields. Moreover, there is a hint that a fundamental constraint on ape mentality might simply be a shortage of working memory, an idea I would like to have seen further explored.
For Suddendorf, the two features he points to constitute “the gap” between human and ape mentality. To me it appears more like a chasm (remember I am a killjoy). While this chasm must have been opening up throughout the 65 million years of primate evolution, the full extent was only reached approximately six million years ago, the time of the divide between the lineages that led to the modern-day chimpanzee and human. Ultimately the gap must derive from differences in the genomes, and here we return to Pääbo’s “dirty little secret” that we cannot currently connect our knowledge of the genome with the particular characteristics of individual humans.
We do, however, learn something about the differences between the human and ape genomes from Pääbo’s book. He recounts a conference presentation by Corey McLean in 2010 that described how modern humans were missing 583 large chunks of DNA that are present in apes. Those chunks had included some genes that are consequently now absent in humans. One such gene had been related to a protein that limits the extent to which neurons divide and hence its absence has “something to do” with how brains got larger in human evolution. According to these books, that seems to be as far as we can currently go in relating molecular biology to mentality in human evolution.
Fortunately, another chunk of missing DNA provides something more concrete. In the DNA that humans no longer have there was a gene that, in apes, encodes for a protein expressed in penile spines, the curiously named structures on the penises of apes that cause males to ejaculate very quickly. Lacking such spines, modern humans are able to enjoy prolonged intercourse. This gene was also found—luckily for them—to be absent from Neanderthals.
Reading Pääbo’s and Suddendorf’s books together, the obvious question arises of whether the modern gap in mentality between humans and apes is the same in kind as that between modern humans and Neanderthals. When compared to modern humans, were the Neanderthals also limited in their capacity for building mental scenarios and connecting their minds together? Or had these critical abilities for modern human thought already arisen in human evolution, at some time between the six-million-year-old human–ape common ancestor and the 0.4-million-year-old modern human–Neanderthal common ancestor? Suddendorf attempts to address this question in his penultimate chapter but is hindered by the sheer complexity of the archaeological data.
He gets into a tangle. On one page he suggests that dental evidence indicates Homo erectus had enhanced behavioral flexibility in comparison with its ancestors (a rather dubious inference) and may have made “serious headway in both key aspects of the gap.” But then he is confronted by the stasis in technology for more than a million years and asks why Homo erectus did not regularly improve the design of their tools: presumably because they could neither imagine nor talk about new types of artifacts. Homo erectus then somehow becomes clever again by dispersing throughout much of the Old World, i.e., Europe and Asia. These apparent contradictions in Homo erectus’s mental capacity suggest to me that the frame proposed by Suddendorf is missing something fundamental for understanding cognitive evolution, something that only becomes apparent from the archaeological record and thus is largely invisible to psychologists who rely upon observing living, breathing humans and apes.
The “prudent and cautious” analysis that Suddendorf brings to the psychological evidence is lost when it comes to the Neanderthals. I don’t blame him, since the evidence appears even more contradictory than that for Homo erectus. What is unfortunate, however, is that Suddendorf doesn’t cite any of the numerous recent books on Neanderthal mentality; that by Thomas Wynn and Frederick Coolidge would have supported his own hint about the overall significance of working memory capacity.*
The archaeological interpretations are incidental to Suddendorf’s book, which provides the most comprehensive comparison of the mentalities of humans and apes that one can imagine. It is difficult to conceive of any new experiments or field observation that might either challenge or further develop his conclusions. In contrast, although Pääbo’s book ends in 2010, one requires a mere glancing acquaintance with more findings to know that his scientific revolution continues. Just a few months ago, on December 4, 2013, DNA was reported as having been extracted from a 400,000-year-old human fossil. This is ten times older than the Neanderthals and opens up yet another new frontier in human evolution studies.
The fascination with who we are and what makes us different from our close relatives, whether extant or extinct, will continue. It is an alluring subject for everyone from the scientist requiring millions of dollars of research funding to those who sit daydreaming in armchairs. Within a few months of the Neanderthal genome being published, forty-seven people had written to Pääbo claiming that they were Neanderthals; tellingly, forty-six of these were men. Twelve women had also written, suggesting that their husbands were Neanderthals. One may well have been my wife.