Lone Survivors (44 page)

Fortunately this is a fast-moving area of science, and a lot of new data will be arriving to resolve this question in the next few years—including a thousand complete human genomes from around the world. Pardis Sabeti, as well as singing in a rather good rock band, has worked with her colleagues on a new method that combines three tests for multiple signals of selection, and which has the potential to increase the resolution of scans for recently selected DNA as much as a hundredfold. She is also researching something important that we haven't touched on—not every genetic change involves our DNA. Ribonucleic acid (RNA), like DNA, consists of long chains of nucleotides, but these chains are usually single-stranded in our cells. Different types of RNA are central to protein synthesis and to the regulation of gene expression, and thus RNA—which also mutates—forms another subject and agent of evolutionary change. This is part of a growing body of data concerning inheritance that lies beyond the genetic code of DNA, constituting the field of epigenetics (from the Greek, meaning “over or above genetics”). This is a fast-developing area of research that will not replace the current focus on DNA, but certainly provides additional ways of looking at inheritance and evolution. Here, short-term environmental changes may have an impact on bodily form and function beyond changes purely in our DNA—for example, via histone proteins that make up part of the chromosomes, or through the modifications that viruses or prions may inflict on us.

Finally, although this discussion of recent changes in human DNA has constantly referred to selection, we should bear in mind that selective changes may not benefit everyone; there can be winners and losers, as there has been with the rise of sickle-shaped cells in the blood of African-derived populations. Sickling has benefited those who are heterozygous for the sickle-cell gene (that is, they have only one copy of it) by conferring some immunity against the malarial parasite. But without medical intervention, those born with two copies of the gene will be highly anemic and will die prematurely. The frequency of a mutation in the leptin receptor gene has increased dramatically in East Asia, linked with changes in the body mass index and a tendency to store fat. This may have been beneficial for adaptation to colder climates but now is a cause of high blood pressure and obesity. Some researchers have also argued that long and stressful sea voyages, whether forced in the case of the slave trade or voluntary in the case of the colonization of Polynesian islands, would have selected physiques and physiologies that were best able to survive the rigors of those journeys. The survivors then went on to found much larger populations who now live under very different conditions, perhaps explaining the prevalence of salt-sensitive hypertension in American blacks, and of diabetes and obesity in parts of Oceania. Similarly, as the anthropologist Peter Ellison pointed out, it is possible that the apparent increasing frequency of conditions like autism, schizophrenia, allergies, asthma, autoimmune diseases, and reproductive cancers are the modern downside of genetic changes that were beneficial under more ancient human environments and lifestyles. These comparisons between the past and the present are the basis of a whole new field of science called
evolutionary medicine
.

It is not always clear what the precise agent of selection has been in the past, beyond differential reproductive success. Where disease is concerned, it is obvious that this will cause direct natural selection through the reduced fertility or death of those whose natural (inherited) defenses are unable to cope with the condition in question. But exposure to the pathogens may be reduced or increased by particular human behaviors (think of the use of condoms, which act as contraceptives, but also combat the spread of HIV). Thus many of these changes probably lie in the realm of complex interactions between the natural environment and ones we have created through our diversity of human cultures. And this brings us back to one of Darwin's favored evolutionary mechanisms, as highlighted by the full title of his second most famous book:
The Descent of Man, and Selection in Relation to Sex
. It is evident that, as Darwin proposed, some of these changes could be ascribed to human sexual/cultural selection, where habitual preferences in mating could steer evolution in a particular direction. This might well include some of the regional (“racial”) differences in appearance, as Darwin suspected, and equally some of the changes in brains and behavior. Stature is a case in point; it is a complex trait, but one with a high heritability. There is evidence that stature (as long as it does not become excessive) is linked to both fecundity and wealth in the developed world, and studies of the selection of sperm donors suggest that women prefer taller donors, which will in turn lead to taller offspring.

All of this would have fascinated Darwin. When he was alive, the hard data about our origins could have been packed up in a small suitcase, and while in many ways he started the writing of the book of human evolution, all he could achieve was the equivalent of drafting some chapter titles and words and sentences scattered through it. Since then we have learned so much about our early history, and many more words, sentences, and paragraphs of our story are now in place. Some chapters are fairly complete, such as the ones about building complete human and chimpanzee genomes, with the Neanderthal and Denisovan chapters following now. And yet the writing of other chapters has hardly begun, such as the ones about how our brains really work, who were the first peoples of the Indian subcontinent, the early history of the Hobbit in southeast Asia, and who was living in West Africa for most of prehistory.

Certainly, until we have a dated fossil, archaeological, and environmental record from many more regions to match the quality of the ones we have from western Europe, and are beginning to have from places like eastern and southern Africa, we cannot even guess how the book of our evolutionary history will look when it nears completion. Paleoanthropology is such a fast-moving and fast-developing science that even some of what is already written in that book will need to be corrected, or perhaps even deleted altogether, including my own contributions, no doubt. The process of writing this book has led me to a greater recognition of the forces of demography, drift, and cultural selection in recent human evolution than I had considered before. And while I have been writing it, new genetic data have emerged to show that we
Homo sapiens
are not purely derived from a recent African origin. But this dynamism is what makes studying human evolution so fascinating, and science is not about being right or wrong, but about gradually approaching truth about the natural world.

When Darwin died and was granted the honor of being buried in Westminster Abbey, there were many rich tributes to the man and his work, as this example shows.

Mr. Darwin has left as broad and deep a mark upon Psychology as he has upon Geology, Botany, and Zoology. Groups of facts which previously seemed to be separate, are now seen to be bound together in the most intimate manner; and some of what must be regarded as the first principles of the science, hitherto unsuspected, have been brought to light. If the proper study of mankind is man, Mr. Darwin has done more than any other human being to further the most desirable kind of learning, for it is through him that humanity in our generation has first been able to begin its response to the precept of antiquity—know thyself.

That last phrase harks back to ancient Greece but was also Linnaeus's directive in describing the species he named
Homo sapiens
. Knowing thyself, for me, has meant a journey from measuring fossil skulls in European museums forty years ago to looking at almost every aspect of our origins. Knowing ourselves has meant recognition that becoming “modern” is the path we perceive when looking back on our own evolutionary history. That history seems special to us, of course, because we owe our very existence to it. Those figures of human species (usually males) marching boldly across the page have illustrated our evolution in many popular articles, but they have wrongly enshrined the view that evolution was simply a progression leading to us, its pinnacle and final achievement. Nothing could be further from the truth. There were plenty of other paths that could have been taken; many would have led to no humans at all, others to extinction, and yet others to a different version of “modernity.” We can only inhabit one version of being human—the only version that survives today—but what is fascinating is that paleoanthropology shows us those other paths to becoming human, their successes and their eventual demise, whether through failure or just sheer bad luck. Sometimes the difference between failure and success in evolution is a narrow one, and we are certainly on a knife edge now as we confront an overpopulated planet and the prospect of global climate change on a scale that humans have never faced before. Let's hope our species is up to the challenge.

Sources and Suggested Reading

General Reading

Boyd, R., and J. Silk.
How Humans Evolved
. 5th ed. Norton, New York, 2009.

Cartmill, M., and F. Smith.
The Human Lineage
. 2nd ed. Wiley-Blackwell, Hoboken, NJ, 2009.

Darwin, C. The Complete Work of Charles Darwin Online.
http://darwin-online.org.uk
.

Fagan, B.
Cro-Magnon: How the Ice Age Gave Birth to the First Modern Humans
. Bloomsbury Press, London, 2010.

Johanson, D., and K. Wong.
Lucy's Legacy: The Quest for Human Origins
. Harmony Books, New York, 2009.

Klein, R. G.
The Human Career.
University of Chicago Press, Chicago, 2009.

Lewin, R., and R. A. Foley.
Principles of Human Evolution
. Blackwells, Oxford, 2003.

Lockwood, C.
The Human Story
. Natural History Museum, London, 2007.

Potts, R., and C. Sloan.
What Does It Mean to Be Human?
National Geographic, Washington, DC, 2010.

Stringer, C.
Homo britannicus
. Allen Lane, London, 2006.

_____
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357 (2002), 563–79.

Stringer, C., and P. Andrews.
The Complete World of Human Evolution
. Thames & Hudson, London, 2005.

Stringer, C., and C. Gamble.
In Search of the Neanderthals
. Thames & Hudson, London, 1993.

Wood, B.
Human Evolution (a Brief Insight)
. Sterling, New York, 2011.

Zimmer, C.
Smithsonian Intimate Guide to Human Origins
. Harper, New York, 2007.

1. The Big Questions

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South African Journal of Science
74 (1978), 409–19.

Brace, C. L. “The fate of the ‘Classic' Neanderthals: A consideration of hominid catastrophism.”
Current Anthropology
59 (1964), 3–43.

Bramble, D. M., and D. E. Lieberman. “Endurance running and the evolution of
Homo
.”
Nature
432 (2004), 345–52.

Bräuer, G. “The ‘Afro-European
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hypothesis' and hominid evolution in east Asia during the Middle and Upper Pleistocene.”
Courier Forschungsinstitut Senckenberg
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Campbell, B. G. “Conceptual progress in physical anthropology: Fossil Man.”
Annual Review of Anthropology
1 (1972), 27–54.

Cann, R. L., M. Stoneking and A. C. Wilson. “Mitochondrial DNA and human evolution.”
Nature
329 (1987), 111–12.

Cavalli-Sforza, L. L., A. Piazza, P. Menozzi and J. Mountain. “Reconstruction of human evolution: Bringing together genetic, archaeological, and linguistic data.”
Proceedings of the National Academy of Sciences USA
85 (1988), 6002–6.

Clark, D. “Africa in prehistory: Peripheral or paramount?”
Man
10 (1975), 175–98.

Coon, C. S.
The Origin of Races
. Alfred A. Knopf, New York, 1962.

Dart, R. A. “
Australopithecus africanus
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Nature
115 (1925), 195–99.

Darwin, C.
The Descent of Man, and Selection in Relation to Sex
. John Murray, London, 1871; Penguin Classics, London, 2004.

_____
.
On the Origin of the Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life
. John Murray, London, 1859.

Gröning, F., J. Liu, M. J. Fagan and P. O'Higgins. “Why do humans have chins? Testing the mechanical significance of modern human symphyseal morphology with finite element analysis.”
American Journal of Physical Anthropology
(2010) (doi: 10.1002/ajpa.21447).

Howell, F. C. “Upper Pleistocene men of the southwest Asian Mousterian.” In G. H. R. von Koenigswald, ed.,
Neanderthal Centenary, 1856–1956
, pp. 185–98. Kemink en Zoon, Utrecht, 1958.

Howells, W. W. “Explaining modern man: Evolutionists versus migrationists.”
Journal of Human Evolution
5 (1976), 477–96.

Hrdli
č
ka, A.
The Skeletal Remains of Early Man
. Smithsonian Institution, Washington, DC, 1930.

Klein, R. G.
The Human Career
. University of Chicago Press, Chicago, 2009.

Leakey, R. E. F., K. W. Butzer and M. H. Day. “Early
Homo sapiens
remains from the Omo River region of Southwest Ethiopia.”
Nature
222 (1969), 1132–38.

Linnaeus, C.
Systema Naturae
. 10th ed. Vol. 1. Holmiae, Salvii, 1758.