On the ancestry of the horse (Equus)

in #hypothesis6 years ago (edited)

It is known that the Equus lineage of horses that are alive today has a common ancestor around 4.4 million years ago (Orlando, L. 2013, Ryder, O.A. 1986). It is also known that the fossil record in Afar shows a trend towards grazing as a niche (Haile-Selassie, Y. 2015), a result of that those niches speciated on the coastal grasslands of Danakil (La Lumiere, L.P. 1981, Nygren, J. 2018). Donald Johanson, who discovered the famous Au. afarensis fossil specimen "Lucy", mentions in Lucy: The Beginnings of Humankind from 1981 that there are Equus teeth from below the KBS tuff at Koobi Fora, dated to nearly three million years, causing Johanson to re-adjust his estimate for the age of the KBS tuff to 1.8 million years instead of 3 million years (Johanson, D. 1981).

Perhaps it was not the fossil record that was false, but rather, the a priori model for where the Equus lineage originated, and the reason Eurygnathohippus afarense, that shows adaptations towards increased dependence on grazing, with only cranial remains to go by, have not been identified outside Hadar (Gilbert, W. 2009), is because it is not recognized as ancestral to Equus that is abundant throughout East Africa in the Pleistocene, including the late Pliocene in the 3 million year old strata at Koobi Fora (Johansen, D. 1981).

Synapses

Orlando, L., Ginolhac, A., Zhang, G., Froese, D., Albrechtsen, A., Stiller, M., … Willerslev, E. (2013). Recalibrating Equus evolution using the genome sequence of an early Middle Pleistocene horse. Nature, 499(7456), 74–78. https://doi.org/10.1038/nature12323

George, M. Jr. Ryder, O.A. (1986). Mitochondrial DNA evolution in the genus Equus. Molecular Biology and Evolution. https://doi.org/10.1093/oxfordjournals.molbev.a040414

Levin, N. E., Haile-Selassie, Y., Frost, S. R., & Saylor, B. Z. (2015). Dietary change among hominins and cercopithecids in Ethiopia during the early Pliocene. Proceedings of the National Academy of Sciences, 112(40), 12304–12309. https://doi.org/10.1073/pnas.1424982112

La Lumiere, L. P. “Evolution of Human Bipedalism: A Hypothesis About Where It Happened.” Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, vol. 292, no. 1057, 1981, pp. 103–107. JSTOR, JSTOR, www.jstor.org/stable/2398648.

Nygren, J. (2018). On the Origin of the Species, Danakil as the Galapagos of Pliocene East Africa, https://gist.github.com/resilience-me/1ee3b88c977ec5f8f46e08ffe42fd92a

Johanson, D. (1981). Lucy: The beginnings of humankind. New York: Warner Books. 409 pp.

Gilbert, W. H., & Bernor, R. L. (2009). Equidae. In Homo erectus: Pleistocene Evidence from the Middle Awash, Ethiopia (pp. 133–166). University of California Press. https://doi.org/10.1525/california/9780520251205.003.0006

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