In the six million years since we shared a common ancestor with the two chimpanzee species living today, our family branch has changed in many obvious ways. At any one time, there were usually several related species.[i] Just a hundred thousand years ago, Homo sapiens, Homo Neanderthalensis and Homo erectus co-existed, and only forty thousand years ago it appears that at least three other species shared the Earth with us, including the Neanderthals, the Denisova hominin in the Siberian Altai Mountains, and the diminutive Homo floresiensis on the Indonesian island of Flores.[ii] Since the extinction of Neanderthals about thirty-thousand years ago, we have tread the world alone as the only big-brained bipedal primate.[iii] Given the paucity of archeological data, we don’t know exactly which of the many hominid skeletons belong to our ancestral line.[iv] However even in the sparse fossil record we can read of an evolution of body plan and brain size that increasingly matches our own.[v] Humans have not always walked the Earth in their present form. We are products of an evolutionary history.
Image credit: Creative Commons Attribution Share-Alike 2.5 License
Much of our evolution occurred during the Pleistocene, the geological epoch that began about 2.6 million years ago and ended during the beginning of the last interglacial period about twelve thousand years ago. Glacial-interglacial cycles played climatic ping-pong on the African continent, our home for most of that time.[vi] Dry savannah alternated with steamy jungle. Likewise, the kinds of animals and plants varied, as did floods and drought, heaving rivers and dry lakes.[vii] We became bipedal, hairless, and bigger brained.[1] Still, the exigencies of climate change almost exterminated us. Sometime between seventy thousand and hundred-fifty thousand years ago humanity had bottlenecked to a population of perhaps just hundreds, as this small surviving group was driven by droughts and deserts into the bottom of Southern Africa.[viii] Our species could have become extinct, going the way of most species, but we hung on tenaciously along the South African coast for nearly a hundred thousand years, finding ways to adapt to our new coastal habitat.[ix] What carried us through were innovations that came from a punctuated jump in brain size and a big bang in cultural complexity.[x] Without them we would have likely gone the way of the rest of our ancestors. Instead, we survived and spread and gradually took over the world.
About fifty thousand years, humans migrated across the face of the Earth, and the human genome encountered a wild variety of new worlds—from the Himalayan foothills and arctic snowfields to the Amazonian rainforests and Bangladesh floodplains.[xi] And humans changed in accordance.[xii] The color and texture of skin, hair, and eyes changed in geographical isolated populations.[xiii] The Tibetans branched off from the Han Chinese about three thousand years ago and have since evolved several genes that facilitate breathing the low-oxygen air of high altitudes.[xiv] In Africa there are some twenty genes that code for traits such as the sickle cell that serve as protection against malaria.[xv] And in Nigeria a mutation has been found in the Yoruba people biologically linked to the virus that causes the terrible Lassa fever.[xvi] Many of our genes seem to be brand new as they code for the metabolism of a diet that didn’t exist until the domestication of plants and animals just ten thousand years ago. Lactose tolerance evolved independently in Africa and Europe as a response to the relatively new habit of breeding and penning animals for their milk and meat.[xvii] People in farming societies have more copies of the AMY1 gene than do most hunter-gatherers.[xviii] This gene probably codes for the amylase protein in the human saliva that breaks down the starch in civilization’s grain-heavy diets. In less than four hundred generations, the human genome has undergone some two thousand new genetic adaptations, affecting every human body system. Jaw size, bone and tooth density, life span, immunity to pathogens, sperm production, brain structure, and even our sense of taste and smell have changed in response to our new environments.[xix]
One newly discovered genetic adaptation helps Tibetans produce more folate, which aids in pregnancy but can become depleted at high altitudes.
KEVIN FRAYER/GETTY IMAGES
Our success has been reflected in our expanding population, which in turn led to even greater success for our species. Larger numbers meant more reproduction and correspondingly more chances for genetic mutation. Most changes in the DNA code are either fatal or neutral, but some small percent confers an advantage to the individual and to her descendants.[xx] And denser populations increased genetic vigor, as did animal husbandry and a settled—as opposed to a nomadic—way of life. Each contributed in providing ideal breeding grounds for pathogens. Disease and early death became a common feature of human existence, culling out the “weaker” individuals and selecting for strong immune systems. Note the near collapse of many of the Native American populations when they encountered the pathogens that had been brewing for millennia in the cities and farms of the Old World.[xxi] Population density also facilitated the development of social intelligence, inhibition (the ability to delay gratification), and the control of urges such as anger, desire, and lust.[xxii] These neurological functions are located in the prefrontal cortex, the newest and most neuron dense portion of our brain.[xxiii]
This 16th century copper plate engraving by Theodor de Bry depicts Christopher Columbus landing in the Caribbean.
Courtesy of the University of Florida Smathers Library Special Collections
Since most mutations occur during the process of cell reproduction, faster reproducing species have an advantage in fast-changing conditions. We, however, are a slow-reproducing species. A few offspring every twenty-five years or so. That’s four generations, four roll-of-the-die, every one hundred years. Like all big mammals, rapid adaptation was not our strong suit.
But the enlarged, more complex brain transcended these biological constraints.[xxiv] With it emerged a host of sophisticated cognitive capabilities, including language, reasoning, forethought, abstraction, memory, imitation and meta-cognition (that is, the awareness of cognizing).[xxv] The new human brain could learn new tricks in a minute’s time and thus change eon-long behaviors in the blink of an eye. This one adaptation of increased encephalization compensated for thousands of other possible anatomical changes. If the climate turned cold, we stole an animal’s fur and built an insulated shelter; if desertic, we dug wells until we hit water; where advantageous, we settled down and domesticated plants and animals; we burned down forests to create grassland game parks; we fashioned faster, stronger weapons; we diverted streams and waterways and drained unwanted swamps; we built ships to colonize new lands and to hunt whales and fish; we mined metals and hardened them into all manner of tool and weapon. Millions of years in the African crucible had fashioned a highly adaptable animal. As Spencer Wells put it, “we are biologically adapted to adapt.” With nimble minds we copy, innovate, and scheme; with bodies designed to do most anything we act upon the world.
ENDNOTES
[1] The bipedal human uses a quarter the energy of a chimpanzee when walking. Our skin lost hair and gained sweat glands, allowing us an unusual stamina in stalking prey.
[i] Wood, B., and Harrison, T. (2011) The Evolutionary Context Of The First Hominins, Nature, v. 470, pp. 347-352.
[ii] Stringer (2003), Brown et al. (2004), Morwood et al. (2004), Kraus et al. (2007), Reich et al. (2010), Neimark (2011), Stewart and Stringer (2012).
Stringer, C. (2003) Out of Ethiopia, Nature, v. 423, pp. 692-694.
Brown, P. Sutikna, T., Morwood, M.J., Soejono, R.P., Jatmiko, Wayhu Saptomo, E., and Rokus Awe Due (2004). A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia. Nature, v. 431, pp. 1055–1061.
Morwood, M.J., Soejono, R.P., Roberts, R.G., Sutikina,T., Turney, C.S.M., Westaway, K.E., Rink, W.J., Zhao, J., van Den Bergh, G.D., Rokus Awe Due, Hobbs, D.R., Moore, M.W., Bird, M.I., and Fifield, L.K. (2004) Archaeology and age of a new hominin from Flores in eastern Indonesia. Nature, v. 431, pp. 1087–1091.
Krause, J., Orlando, L., Serre, D. Viola, B., Prüfer, K., Richards, M.P., Hublin, J.J., Hänni, C., Derevianko, A.P., and Pääbo, S. (2007) Neanderthals in central Asia and Siberia, Nature, v. 449, pp. 902-904.
Reich, D., Green, R.E., Kircher, M., Krause, J., Patterson, N. Durand, E.Y, Viola, B., Briggs, A.W., Stenzel, U., Johnson, P.L.F., Maricic, T., Good, J.M., et al. (2010) Genetic history of an archaic hominin group from Denisova Cave in Siberia, Nature, v. 468, pp. 1053-1060.
Neimark, J. (2011, May) Meet the New Human Family, Discover, pp. 48-55,76.
Stewart, J.R., and Stringer, C.B. (2012) Human Evolution Out of Africa: The Role of Refugia and Climate Change, Science, v. 335, pp. 1317-1321.
[iii]Krause et al. (2007)-citation above. Finlayson, C., and Carrion, J.S. (2007) Rapid Ecological Turnover And Its Impact On Neanderthal And Other Human Populations, Trends in Ecology and Evolution, v. 22(4), pp. 213-222.
[iv] Wood and Harrison (2011)—citation above.
[v] Tattersall, I. (2009) Human origins: Out of Africa. Proceedings of the National Academy of the Sciences United States of America (PNAS), v. 106, pp. 16018–16021.
[vi] Potts (1998), Proctor (2002), Wells (2003), Scholz et al. (2007) Tattersall (2009)—citation above, Armitage et al. (2011), deMenocal (2011), Stewart and Stringer (2012)—citation above.
Potts, R. (1998) Environmental Hypotheses of Hominin Evolution, Yearbook of Physical Anthropology, v. 41, pp. 93-136.
Proctor, R.N. (2002) Out of the Chattering Ice, book review. Science, v. 296, pp. 2342-2343.
Wells, S. (2003) The Journey of Man: A Genetic Odyssey. Random House, New York.
Scholz, C.A., Johnson, T.C., Cohen, A.S., King, J.W., Peck, J.A., Overpeck, J.T., Talbot, M.R., Brown, E.T., Kalindekafe, L., Amoako, P.Y.O., Lyons, R.P., Shanahan, T.M., Castañeda, I.S., Heil, C.W., Forman, S.L., McHargue, L.R., Beuning, K.R., Gomez, J., and Pierson, J. (2007) East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins, Proceedings of the National Academy of the Sciences of the United States of America (PNAS), v. 421, pp. 16416-16421.
Armitage, , S.J., Jasmin, S.A., Marks, A.E., Parker, A.G., Usik, V.I., and Uerpmann, H.P. (2011) The Southern Route ˙out of Africa”: Evidence for an Early Expansion of Modern Humans into Arabia. Science, v. 331, pp. 453-456.
deMenocal, P.B. (2011) Climate and Human Evolution. Science, v. 331, pp. 540-542.
.[vii] Wells (2003), deMenocal (2011), Stager et al. (2011).
Stager, J.C., Ryves, D.B., Chase, B.M., and Pausata, F.S. (2011) Catastrophic Drought in the Afro-Asian Monsoon Region During Heinrich Event 1. Science, v. 331, pp. 1302.
[viii] Marean, C.W. (2010a, August) When the Sea Saved Humanity, Scientific American, pp. 55-61. Another possible bottleneck occurred around 74,000 years ago with the eruption of Mount Toba (Rampino and Ambrose, 2000; Jones, 2013).
Rampino, M.R., and Ambrose, S.H. (2000) Volcanic Winter in the Garden of Eden: The Toba Supereruption and the Late Pleistocene Population Crash. Geological Society of America Special Paper, pp. 71-82 in McCoy, F.W., and Heiken, G. (Editors) Volcanic Hazards and Disasters in Human Antiquity, Geological Society of America Special Paper 345, Inc., Boulder, CO.
Jones, N. (2013) It Could Happen One Night: Catastrophes from the Past Will Strike Again—We Just Don’t Know When, Nature, v. 493, pp. 154-156.
[ix] Marean et al. (2007), Marean (2010a).
Marean, C.W. (2010a, August) When the Sea Saved Humanity, Scientific American, pp. 55-61.
[x] Marean (2010b), Culotta (2010). That there was a punctuated jump isn’t in question. For some the timing of this big bang occurred later. For example, see Coolidge and Wynn (2001, 2005), Wells (2003), Diamond (1999), Balter (2010b.
Marean, C.W. (2010b) Coastal South Africa and the co-evolution of the modern human lineage and the coastal adaptation, in Trekking the Shore: Changing Coastlines and the Antiquity of Coastal Settlement, edited by Nuno Bicho, Jonathan A. Haws, and Loren G. Davis, Springer, New York.
Culotta, E. (2010) Did Modern Humans Get Smart or Just Get Together, Science, v. 328, p. 164.
Wynn, T. (2002) Archaeology and Cognitive Evolution, Behavioral and Brain Sciences, v. 25, pp. 389-438.
Wynn, T., and Coolidge, F. (2008) Why not Cognition? Current Anthropology, v. 49, pp. 895.
Diamond, J. (1999) Guns, Germs and Steel: The Fates of Human Societies, W.W. Norton and Company, New York
Balter, M. (2010) Did Working Memory Spark Creative Culture, Science, v. 328, pp. 160-163.
[xi] Diamond (1999), Forster (2004), Pritchard (2010).
Forster, P. (2004) Ice Ages and the Mitochondrial DNA Chronology of Human Dispersals: a Review. Philosophical Transactions of the Royal Society of London B, v. 359, pp. 255-264.
Pritchard, J.K. (2010, October) How We are Evolving. Scientific American, pp. 41-47.
[xii] Cochran and Harpending (2009), McAuliffe (2009), Ward (2009), Pritchard (2010).
McAuliffe, K. (2009, March) Are We Still Evolving? Discover, pp. 51-56, 58.
[xiii] McAuliffe (2009), Ward (2009), Laland, Odling-Smee and Myles (2010), Pritchard (2010).
Laland, K.N., Odling-Smee, J., and Myles, S. (2010) How Culture Shaped the Human Genome: Bringing Genetics and the Human Sciences together, Nature Reviews, v. 11, pp. 137-148.
[xiv] Pritchard (2010).
[xv] Sabeti (2008), Cochran and Harpending (2009), McAuliffe (2009).
[xvi] Sabeti, P. (2008) Natural Selection: Uncovering Mechanisms of Evolutionary Adaptation to Infectious Disease. Nature Education, v. 1(1).
[xvii] Cochran and Harpending (2009), Laland et al. (2010), Odling-Smee and Myles (2010).
[xviii] Perry et al. (2007).
[xix] McAuliffe (2009), Ward (2009), Laland, Odling-Smee and Myles (2010), Wade (2010, March 1).
[xx] Sawyer S.A., Parsch, J., Zhang Z, Hartl, D.L. (2007). Prevalence of Positive Selection Among Nearly Neutral Amino Acid Replacements in Drosophila. Proceedings of the national Academy of the United States of America, v. 104 (16): 6504–6510.
[xxi] Balter, M. (2011) Genes Confirm European’s Blow to Native Americans, Science, v. 334, p. 1335..
[xxii] “Tasks of inhibition” were key in the development of trapping animals, some forms of hunting and agriculture as survival strategies and are unknown in the archeological record prior to the Upper Paleozoic (Coolidge, F. L. and Wynn, T. (2001). Executive Functions of the Frontal Lobes and the Evolutionary Ascendancy of Homo sapiens, Cambridge Archaeological Journal, v. 11, 255-260.)
[xxiii] Purves et al. (2007), Coolidge and Wynn (2001).
Purves, D., Augustine, G.J., Fitzpatrick, D., Katz, L.C., LaMantia, A-S., McNamara, J.O. (Editors) (1997) Neuroscience. Sinauer Associates, Inc. Publishers, Sunderland, MA
[xxiv] Indeed, as Pritchard (2010) noted, “For a number of the challenges facing our species… natural selection probably occurs too slowly to help us much. Instead we are going to have to rely on culture and technology.”
[xxv] Coolidge and Wynn (2001, 2005).
