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Hominin Evolution

November 14, 2022

The relative unimportance of Earth on a universal scale has been accentuated recently by the first deep field image of the James Webb Space Telescope. As if that wasn't ego deflating enough, a recent study estimates that the ant population of the Earth is 20 quadrillion; that is, 20 x 1015.[1-2] This estimate means that there are about 2.5 million ants for every human. This reminds me of a cartoon in which space aliens deduced from their observations of Earth that the automobile was its dominant species.[3]

Driver ants of the genus Aenictus

Africa and Australasia are the home of driver ants of the genus Aenictus. These ants move quickly through forests in columns of thousands.

These particular ants were photographed on the Lockhart River in Queensland, Australia.

(Portion of a University of Würzburg image)

Nature has surprises other than the vast ratio of the number of ants to humans. Birth gender in humans is determined at conception, but the sex of most turtles, crocodiles, alligators, and teleost fish depends on the temperature at which their eggs develop.[4-5] This temperature-dependent sex determination means that turtle eggs incubated below 27.7°C (81.86°F) produce male turtles, while female turtles will be produced above 31°V (88.8°F). A mix of sexes will be produced when temperatures fluctuate between these extremes.[4] Global warming will skew turtle populations towards a higher female to male ratio.

Temperature-dependent sex determination differs from genetic sex determination, but there's no question that Earth's climate has induced changes in evolution from extinction and genetic mutation driven by adaptational to new climatic conditions. The Chicxulub impact event about 66 million years ago caused the demise of 75% of living species and the eventual extinction of the dinosaurs. It also allowed the rise of mammals, perhaps aided by a diversity of plant species.[6] Closer to our time, climate change, such as the ice ages, has affected species extinction.[7]

There have been five major ice ages, the earliest of these occurred about two billion years ago, long before the advent of humans. The most recent ice age started about 2.6 million years ago, and we are presently in an interglacial period of that age that began about 10,000 years ago. Since modern humans, Homo sapiens, emerged about 300,000 years ago, our ancestors were alive during a glacial period, as were our cousin species, the Neanderthals in Europe and the Denisovans in Asia. The Neanderthals and the Denisovans appear to have gone extinct before the end of the glacial period. Humans arrived in North America about 23,000 years ago, a time close to the peak of the glaciation.

Species adapting to climate change

Species known to be adapting to climate change.[8] From upper left, clockwise, sockeye salmon (Oncorhynchus nerka), thyme (Thymus vulgaris), tawny owl (Strix aluco), red squirrel (Sciurus vulgaris), banded snail (Cepaea hortensis), and fruit fly (Drosophila melanogaster).

(Images: sockeye salmon from the University of Washington, thyme from the New York Public Library, tawny owl by Barry Forbes, red squirrel by Peter Trimming, banded snail by S. Rae, and fruit fly by Alexis Orion, all from Wikimedia Commons. Click for larger image.)

To what extent has climate variation affected human evolution? The demise of the Neanderthals and the Denisovans, possibly related to ice age conditions, was one factor, since it eliminated competition from other hominini. This might be attributed more to better adaptation by humans to climate change than a problem with our Homo cousins. An in-depth study of factors affecting human evolution appeared in a 2010 report by the United States National Research Council Committee on the Earth system Context for Hominin Evolution.[9] As stated in the introduction to this report,
"This fossil record contains a history of critical evolutionary events that have ultimately shaped and defined what it means to be human, including the origins of bipedalism; the emergence of our genus Homo; the first use of stone tools; increases in brain size; and the emergence of Homo sapiens, more advanced tools, and culture. Some of these events appear to have coincided with major changes in African climate, raising the intriguing possibility that key junctures in human evolution and behavioral development may have been climatically mediated."[9]

In 1871, Charles Darwin proposed that the reason that hominins became bipedal was a result of climate change that expanded African grasslands, but this theory still lacks sufficient scientific evidence to be generally accepted.[9] However, the fossil record does contain evidence that climate change was associated not only with the emergence of our genus, Homo, but our first use of stone tools and increased brain size.[9] Genetic mutations beneficial to survival in changing environment will proliferate in a species and become established.[9]

A huge international team of scientists has published an open access paper recently in Nature Geoscience that examines how climate variability influenced hominin evolution in eastern Africa during the Pleistocene, a geological epoch that ended 11,700 years ago.[10-11] Team members are from the University of Cologne (Cologne, Germany), the University of Science and Technology (Palapye, Botswana), Addis Ababa University (Addis Ababa, Ethiopia), the University of Oxford (Oxford, United Kingdom), the University of Minnesota, Duluth (Duluth, Minnesota), Aberystwyth University (Aberystwyth, United Kingdom), the Berkeley Geochronology Center (Berkeley, California), the University of Potsdam (Potsdam, Germany), the University of Liverpool (Liverpool, United Kingdom), the University of Bremen, (Bremen, Germany), Eberhard Karls Universität (Tübingen, Germany), and the University of Cambridge (Cambridge, United Kingdom).[10]

Numerous hominin fossils have been discovered in eastern Africa, but the climatic history of that region concurrent with the creation of these fossils has been unknown.[11] This is particularly true for the Pleistocene (Ice Age) epoch between 2,580,000 to 11,700 years ago.[11] The Hominin Sites and Paleolakes Drilling Project (HSPDP) was started in 2008 to discover the climatic record by means of deep core drilling operations in 2013–2014.[10] This project included more than 22 researchers from 19 institutions in 6 countries, and it was led by Verena Foerster of the Institute of Geography Education at the University of Cologne.[11]

Such a research endeavor needs an interdisciplinary team of geoscientists, sedimentologists, paleontologists, geologists, geographers, geochemists, archaeologists, chronologists, and climate modelers.[11] As reported in their research paper, they extracted two continuous sediment cores from which microfossils and elemental variations could be used as a means to reconstruct climatic history. Archaeologists, evolutionary biologists, and evolutionary anthropologists were then able to identify phases of climatic stress that changed human habitats, influenced human biological and cultural evolution and human diaspora.[11]

The project included five drill sites in Kenya and Ethiopia that are located close to key paleoanthropological sites representing various steps in human evolution.[11] The Chew Bahir drilling site in southern Ethiopia covered the past 620,000 years, a period that includes the time when Homo sapiens emerged in Africa.[10-11] Two 280 meter cores were drilled from that site and their analysis formed the basis of the paper.[10-11]

Recovering core sediments at the hew Bahir drilling site.

Recovering core sediments at the hew Bahir drilling site. In this photo, the day shift team is pulling the liner from a section of the 280 meter long core. (University of Cologne photo by Frank Schäbitz)

The first of three distinct phases of eastern Africa climatic variability coincided with the appearance of high anatomical diversity in hominin groups in relatively stable humid conditions from about 620,000 to 275,000 years ago with occasional abrupt and extreme hydroclimate disturbances.[10-11] The second phase, from about 275,000 to 60,000 years ago cycled between lush vegetation with deep fresh water lakes to highly arid landscapes.[10-11] In that phase, human tools transitioned to more complex Middle Stone Age technologies accompanied by important human social and cultural innovations.[10-11] Says lead author of the research paper, Verena Foerster,
"These innovations, such as more varied toolkits and long-distance transport, would have equipped modern humans with an unprecedented adaptability to the repeated expansions and contractions of habitats."[11]

The third phase, from 60,000 to 10,000 years ago had the most extreme climatic fluctuations and the most arid phase of the entire record.[11] Such conditions could have caused the continuous cultural changes seen in that period.[11] The research team believes that the concurrence of humid peaks in eastern Africa with wet phases in north-eastern Africa and the Mediterranean created favorable migration routes from Africa to facilitate the global dispersal of Homo sapiens.[11] Says Foerster,
"In view of current threats to the human habitat from climate change and the overuse of natural resources through human activity, understanding the relationship between climate and human evolution has become more relevant than ever."[11]


  1. Patrick Schultheiss, Sabine S. Nooten, Runxi Wang, Mark K. L. Wong, François Brassard, and Benoit Guénard, "The abundance, biomass, and distribution of ants on Earth," Proc. Natl. Acad. Sci., vol. 119, no. 40 (September 19, 2022), Article no. e2201550119, https://doi.org/10.1073/pnas.2201550119.
  2. How many ants are there?, University of Würzburg, September 18, 2022.
  3. What on Earth! The Automobile Inherits the Planet, YouTube video by the National Film Board of Canada, posted by NextSTL, July 14, 2016.
  4. What causes a sea turtle to be born male or female?, National Oceanic and Atmospheric Administration (NOAA) website, June 15, 2022.
  5. Environmental Sex Determination, from S.F. Gilbert, "Developmental Biology, 6th edition," (Sinauer Associates, 2000).
  6. Elizabeth Pennisi, "How life blossomed after the dinosaurs died," Science, October 24, 2019, doi: 10.1126/science.aaz9741.
  7. Larisa R. G. DeSantis, Judith H. Field, Stephen Wroe, and John R. Dodson, "Dietary responses of Sahul (Pleistocene Australia–New Guinea) megafauna to climate and environmental change," Paleobiology, vol. 43, no. 2 (May, 2017), pp. 181-195, DOI: https://doi.org/10.1017/pab.2016.50.
  8. Helen Thompson, "Ten Species That Are Evolving Due to the Changing Climate," Smithsonian magazine, October 24, 2014.
  9. Understanding Climate’s Influence on Human Evolution, National Research Council (US) Committee on the Earth system Context for Hominin Evolution, (National Academies Press:Washington (DC), 2010).
  10. Verena Foerster, Asfawossen Asrat, Christopher Bronk Ramsey, Erik T. Brown, Melissa S. Chapot, Alan Deino, Walter Duesing, Matthew Grove, Annette Hahn, Annett Junginger, Stefanie Kaboth-Bahr, Christine S. Lane, Stephan Opitz, Anders Noren, Helen M. Roberts, Mona Stockhecke, Ralph Tiedemann, Céline M. Vidal, Ralf Vogelsang, Andrew S. Cohen, Henry F. Lamb, Frank Schaebitz, and Martin H. Trauth, "Pleistocene climate variability in eastern Africa influenced hominin evolution," Nature Geoscience, September 26, 2022, https://doi.org/10.1038/s41561-022-01032-y. This is an open access article with a PDF file here
  11. Key phases of human evolution coincide with flickers in eastern Africa's climate, University of Cologne Press Release, September 26, 2022.

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