A. From Monkeys to Apes
If you saw your catarrhine ancestors from 30 million years ago, you would most likely call them monkeys. Monkeys are relatively small animals, on average about the size of cats. They run along the tops of large tree branches on all four limbs, palms down, and leap from branch to branch. Their arm and leg joints have limited motion. A monkey’s tail acts as a fifth limb; it is long and prehensile and good at grabbing objects or hanging from branches.
The African catarrhine line of descent diverged into the lines that would become “Old World monkeys” and apes 25 – 30 million years ago. 2 Examples of living Old World monkeys include macaque monkeys, proboscis monkeys, and the unusually large baboons. We are descended from the ape side of the family, which today includes animals ranging in size from the gibbon to the gorilla. The technical term for our earliest ape ancestors is the basal hominoids, but they are just as often called dental apes, because their teeth were among the first features to distinguish them from monkeys.
What are the differences between monkeys and apes? Some differences are a matter of degree: most apes are larger than most monkeys, and ape skulls and brains are relatively larger compared to their bodies. There are other important differences in body structure. Apes do not have tails. Apes have more flexible arms, wrists, and shoulders and more prehensile hands and feet, allowing for a greater range of postures and locomotion than monkeys. When in trees, apes hang downward from branches by their hands and / or feet, and some of them swing or brachiate acrobatically from branch to branch. This last fact is a common point of confusion because we are all familiar with “monkey bars” on the playground. That name is a misnomer. They should be called “ape bars”! All apes can brachiate; Asian apes rely on it as their primary mode of locomotion. 3
The monkey / ape divergence must have been the result of some environmental condition or competition. One leading hypothesis contends that the pressure was scarcity of their favorite food, ripe fruit. Monkeys dealt with this by settling for second best; they evolved sharper teeth and digestive systems for eating leaves and unripe fruit. Apes continued to pursue ripe fruit, but they had to work harder to get it. 4 In particular, apes made more use of their arms in the trees and their legs on the ground. Their skeletal features made them good at climbing, hanging, and swinging by their arms. 5 Their strong, flexible arms and hands were one of their most important contributions to our bodies. 6 Perhaps the absence of a tail favored the evolution of prehensile hands.
Fossil apes continued to share tropical rain forests with monkeys. 7 Because apes were choosier in their diets, though, they were restricted to smaller geographic regions 8 and different niches. Their larger size gave apes a slight advantage in covering more ground between trees.
Full “apification” took tens of millions of years. The oldest known fossil apes lived 20 – 25 million years ago. 9 The earliest species had opposable big toes and had already lost their tails, but otherwise their bodies were still monkey-like. The earliest clear evidence of arm-hanging apes did not appear until 12 million years ago, and brachiation was perfected even more recently. 10
B. The Uniquely Human Clade
Some ape populations migrated out of Africa and spread throughout Europe and Asia at least 17 million years ago. The global temperature was still about 5º C warmer than it is now. 11 Apes’ habitats, dense rain forests, thrived as far north as Europe.
Now that apes occupied a broad geographic range, they evolved into suitably diverse forms. The Neogene was a golden age for apes, with many more species than today. Our ancestors grew larger bodies and brains and became the Linnaean family of hominids or great apes, as distinguished from lesser apes such as today’s gibbons. Only one great ape, the orangutan, survives in Asia today. None survived the Neogene in Europe, as their forest habitat could not survive the cooling climate. The rest of them returned or continued to live in Africa. An interesting debate concerns the question of whether our ancestors were among the temporarily European apes 12 or if they remained in Africa the whole time. 13 This is only one such “Out of Africa” question that arises occasionally in pre-sapiens genealogy. Among the apes that ended up in Africa, three genera have survived to the present. Gorillas branched off from this family tree about ten million years ago.
Then a process occurred that went completely unnoticed at the time, although it seems monumental from today’s perspective. Seven to ten million years ago, 1 14 one particular population of African apes separated into two. One of those sub-populations ultimately evolved into chimpanzees. The other one was the unique forerunner of the human species! Unfortunately, there are few fossils available from that place and time. We do not know exactly what the last common ancestor of chimpanzees and humans looked like. We would not expect it to look just like a modern human or a chimp, since both species have changed since then. The general consensus is that it still lived in trees and looked more like a chimp or a gorilla than a modern human. 15
Except for some overlap in the ranges of gorillas and chimpanzees, the three African apes settled in different regions. Gorillas and chimps inhabited western and central Africa, 16 while human ancestors took to the east and then the south. 17 The features distinguishing gorillas, chimpanzees, and humans were influenced by their habitats. Gorillas, for instance, occupy only a few small patches of rain forest near the equator. Their sharp molars and large guts are adaptations to a diet heavy in leaves. 18 Humans emerged in Africa’s most arid, temperate regions. That process will of course be explored much further below.
Were the East African apes of the late Neogene “humans”? That would be a fair label. Although they looked like animals that we’d put in the zoo today, they were our unshared ancestors. They could certainly not be classified as any other living animal. 2 We are mentally biased, though, by the way humans and other animals appear today. Most of us reserve the word “human” for our more recent ancestors who looked and behaved more like us than like chimpanzees. To satisfy that instinct, our unique ancestors of 2 – 7 million years ago are given the special name “hominins”. It all goes to show that even a word like “human” is truly impossible to define.
- Ape skeleton drawings by Waterhouse Hawkins, c. 1863 (Public domain), https://en.wikisource.org/wiki/Page:Evidence_as_to_Man%27s_Place_in_Nature.djvu/8 (accessed and saved 10/28/19). ↩
- Mark S. Springer et al., “Macroevolutionary Dynamics and Historical Biogeography of Primate Diversification Inferred from a Species Supermatrix”. PLoS ONE 7(11): e49521 (11/16/2012), http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0049521 (accessed and saved 10/20/19). ↩
- Virginia Avis, “Brachiation: The Crucial Issue for Man’s Ancestry”, Southwestern Journal of Anthropology 18(2):119-148 (Summer, 1962), https://www.journals.uchicago.edu/doi/pdfplus/10.1086/soutjanth.18.2.3629013 (accessed and saved 10/20/19). ↩
- L. Alis Temerin and John G.H. Cant, “The Evolutionary Divergence of Old World Monkeys and Apes”, The American Naturalist 122(3):335-351 (Sep., 1983), https://www.journals.uchicago.edu/doi/abs/10.1086/284139?journalCode=an (accessed and saved 10/20/19). ↩
- Russell H. Tuttle, “Chapter 5: Apes in Motion”, Apes and Human Evolution, Harvard University Press (2014). ↩
- David Begun, “Miocene Hominids and the Origins of the African Apes and Humans”, Annual Review of Anthropology, vol. 39 pp. 67-84 at 74 (6/14/2010), https://www.annualreviews.org/doi/abs/10.1146/annurev.anthro.012809.105047 (accessed and saved 10/20/19). ↩
- Russell H. Tuttle, Apes and Human Evolution, Harvard University Press (Kindle eBook version, 2014), location 1151. ↩
- Peter Andrews, “Species diversity and diet in monkeys and apes during the Miocene”, pp. 25 – 61 in Chris Stringer, ed. Aspects of Human Evolution, Taylor & Francis (Boston, 1982). Also see the map by Dennis O’Neil, https://www2.palomar.edu/anthro/primate/prim_6.htm for the range of living Old World monkeys and the map by Peter Sudmant, http://www.washington.edu/news/2013/07/03/great-ape-genetic-diversity-catalog-frames-primate-evolution-and-future-conservation/ for the range of living great apes (both accessed, saved, and archived 10/20/19). ↩
- Nancy J. Stevens et al., “Paleontological evidence for an Oligocene divergence between Old World monkeys and apes”, Nature 497, 611-614 (5/30/2013), https://www.nature.com/articles/nature12161 (accessed and saved 10/20/19). ↩
- Begun, “Miocene Hominids”, op. cit. ↩
- James Hansen et al., “Climate sensitivity, sea level and atmospheric carbon dioxide”, Philosophical Transactions of the Royal Society A 371(2001):20120294 (10/28/2013), https://royalsocietypublishing.org/doi/full/10.1098/rsta.2012.0294 (accessed and saved 10/20/19). ↩
- David Begun is the main proponent for the idea that African great apes descend from those apes that looped into Europe and back in the mid-Miocene. He discusses this hypothesis in numerous papers such as “Miocene Hominids” (op. cit.) and his non-technical book The Real Planet of the Apes: A New Story of Human Origins, Princeton University Press (2016). ↩
- Susanne Cote argues against Begun and defends the traditional view that our ancestry was continuously African throughout the Miocene in “Origins of the African hominoids: an assessment of the paleobiogeographical evidence”, C.R. Palevol 3(4):323-340 (July, 2004), https://www.sciencedirect.com/science/article/abs/pii/S1631068304000855?via%3Dihub (accessed and saved 10/20/19). ↩
- A genetic divergence of 13 MYA is consistent with a genealogical split of 7 – 10 MYA: See comment by John Hawks in Charles Choi, “Human and chimp genes may have split 13 million years ago”, Live Science, 6/12/2014, http://www.livescience.com/46300-chimpanzee-evolution-dna-mutations.html (accessed and saved 2/25/17, archived 10/20/19). ↩
- See e.g. Nathan M. Young et al., “Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees”, PNAS 112(38):11829-34 (9/22/2015), https://www.pnas.org/content/112/38/11829 (accessed and saved 10/20/19). ↩
- Peter Sudmant, “A map showing the geographical distribution of great apes in Africa” in Leila Gray, “Great ape genetic diversity catalog frames primate evolution and future conservation”, UW News (7/03/2013), University of Washington, https://www.washington.edu/news/2013/07/03/great-ape-genetic-diversity-catalog-frames-primate-evolution-and-future-conservation/ (accessed, saved, and archived 10/19/19). ↩
- Kameraad Pjotr and Eric Gaba, “Map of the fossil sites of the early hominids (4.4-1M BP)”, Wikimedia Commons (2009 – 2013), https://commons.wikimedia.org/wiki/File:Map_of_the_fossil_sites_of_the_early_hominids_(4.4-1M_BP).svg (accessed, saved, and archived 10/20/19). ↩
- David Chivers and Claude Hladik, “Morphology of the gastrointestinal tract in primates: Comparisons with other mammals in relation to diet”, Journal of Morphology 166(3):337-386 (Dec., 1980), https://onlinelibrary.wiley.com/doi/abs/10.1002/jmor.1051660306 (accessed and saved 10/20/19). ↩
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