F Rosa Rubicondior: Just a Small Problem - For Creationists

Wednesday 18 October 2017

Just a Small Problem - For Creationists

Lars Gibbon
The last common ancestor of the hominoids may have been about this size.
Research Sizes Up Last Common Ancestor of Humans and Apes:

You know that intermediate form between the old world monkeys and the primates, including humans, that creationists insist never existed?

It weighed about 12 pounds (5.5 Kg) according to a pair of researchers based at the American Museum of Natural History.

As the American Museum of Natural history press release explained:

Among living primates, humans are most closely related to apes, which include the lesser apes (gibbons) and the great apes (chimpanzees, gorillas, and orangutans). These “hominoids” emerged and diversified during the Miocene, between about 23 million to 5 million years ago. Because fossils are so scarce, researchers do not know what the last common ancestors of living apes and humans looked like or where they originated.

To get a better idea of body mass evolution within this part of the primate family tree, [Mark] Grabowski and coauthor William Jungers from Stony Brook University compared body size data from modern primates, including humans, to recently published estimates for fossil hominins and a wide sample of fossil primates including Miocene apes from Africa, Europe, and Asia. They found that the common ancestor of apes was likely small, probably weighing about 12 pounds, which goes against previous suggestions of a chimpanzee-sized, chimpanzee-like ancestor.

Mark Grabowski is a visiting assistant professor at the Eberhard Karls University of Tübingen in Germany working as a post-doctoral fellow at the Natural History Museum's Divisison of Anthroplogy. William L. Jungers is from from Stony Brook University Department of Anatomical Sciences.

They also concluded that the last common ancestor of the hominoids underwent an increase in size to about that of a modern chimpanzee, brought about by adaptive response resource availability and that the australopiths, the immediate ancestors of the Homo genus, shrank somewhat again and were on average smaller than their ancestors. They remained comparatively small until the evolution of Homo erectus.

The research was published last week in an open access paper in Nature Communications:

Body mass directly affects how an animal relates to its environment and has a wide range of biological implications. However, little is known about the mass of the last common ancestor (LCA) of humans and chimpanzees, hominids (great apes and humans), or hominoids (all apes and humans), which is needed to evaluate numerous paleobiological hypotheses at and prior to the root of our lineage. Here we use phylogenetic comparative methods and data from primates including humans, fossil hominins, and a wide sample of fossil primates including Miocene apes from Africa, Europe, and Asia to test alternative hypotheses of body mass evolution. Our results suggest, contrary to previous suggestions, that the LCA of all hominoids lived in an environment that favored a gibbon-like size, but a series of selective regime shifts, possibly due to resource availability, led to a decrease and then increase in body mass in early hominins from a chimpanzee-sized LCA.

Time-calibrated phylogenetic tree with selective regimes and estimated body size optima. a Primate phylogenetic tree including fossils with tips color-coded to denote families, major families noted on the right. Phylogeny showing complete species names shown in Supplementary Fig. 1A Colors along branches showing best-supported selective regimes for body mass evolution including convergence and are consistent between a, b and c. Two major selective regimes for primates and optimal body size for each regime shown on far right correspond to Table 2; b focus on hominoids from a including fossil taxa. Marked nodes correspond to last common ancestors of all hominoids (1), hominids (2), African hominids (3), and chimpanzees and humans (4); c body mass averages (smaller circles) and inferred primary adaptive optima (larger circles) for species in each regime for primates including fossils corresponding to a and b. Numbered LCAs match nodes in b. Also noted is the adaptive optima of chimpanzees, the earliest hominins, later early hominins, and modern humans. Named taxa are outliers to their estimated optima

Alternative hypotheses for primates focused on hominoids. a Best-supported selective regimes with only well-sampled reliably attributed early hominins and without other fossil primates; complete data set with b Brownian motion; c single regime model (OU1); d chimpanzee-sized ancestor all hominoids. Colors reflect regime assignment within each figure and are not comparable between figures

The established wisdom was that our ape ancestors adopted brachiation (swinging, suspended below the branch) as the prime means of locomotion through trees because their body mass became too large for them to run along on top of the branches as the simians do. This was thought to have facilitated bipedalism later on. This finding calls that into question because it seems a small LCA was already developing suspensory locomotion.

The researchers suggest:

...that the ancestor was already somewhat suspensory, and larger body size evolved later, with both adaptations occurring at separate points. The development of suspensory locomotion could have been part of an “arms race” with a growing number of monkey species, the researchers said. Branch swinging allows an animal to get to a prized and otherwise inaccessible food—fruit on the edges of foliage—and larger body would let them engage in direct confrontation with monkeys when required.

While creationism is mired in it's dogma, trying to insist that these facts just aren't there and trying to find arguments to justify ignoring the stark-staringly obvious, proper scientists are working out what these intermediate ancestors were like and how their adaptations contributed later to the physical and behavioural evolution of our evolution and the evolution of our closest relatives.

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