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.
* See Thomas Wynn and Frederick L. Coolidge, How to Think Like a Neanderthal (Oxford University Press, 2011). ↩
See Thomas Wynn and Frederick L. Coolidge, How to Think Like a Neanderthal (Oxford University Press, 2011). ↩