"bushy" human evolution - commentary in Cell

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jim mcnamara
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https://www.cell.com/trends/ecology-evolution/fulltext/S0169-5347(18)30117-4


The authors do not seem to use the term bushy to describe the non-linear evolution of Homo sapiens, but other authors have used it. They use 'African multiregionalism' instead. And also they exclude panmyxia (free for all gene flow as an assumption) in more local populations. The concept is a geographically very widespread series of populations that occasionally interbreed, then remain in isolation, a kind of off again/on again situation. So at any one time in the deep past, there are Homo sapiens populations with very different traits.

Example from other sources: Homo naledi (250kya)and Homo florisiensis (30kya) are extreme examples of isolation of species of Homo concurrent with H. sapiens.

http://humanorigins.si.edu/evidence/human-fossils/species/homo-floresiensis
https://www.newscientist.com/articl...only-250000-years-old-heres-why-that-matters/

The Cell article shows some nice graphics of traits in skulls moving from one population to another.

One takeaway concept is that the nice orderly linear progression model of human evolution we learned in elementary school apparently does not apply well. Lots of branches and dead ends is a better match.
 
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Ygggdrasil
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First, the article was published in the journal Trends in Ecology and Evolution, which is published by Cell Press, but it distinct from the journal Cell.

Debates over the human evolutionary tree have been discussed previously on PF, for example here https://www.physicsforums.com/threads/human-haplogroups-rule-out-humans-descending-from.930013/

Here is a good source on the issue: https://www.nature.com/scitable/topicpage/human-evolutionary-tree-417

In addition to the case for African multiregionalism made by the authors of the commentary piece in Trends in Ecology and Evolution, recent genetic analyses have also found a small percentage of DNA shared between modern Homo sapiens and other hominids found outside of Africa, such as Neanderthals and Denisovans. It remains somewhat unclear whether the mixture represents interbreeding between Homo sapiens and these other hominids or whether the genetic exchanges occurred between ancestors of the different lineages in Africa.

Hopefully more advanced DNA sequencing technologies and better analytical tools can shed further light on this topic.
 
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Ygggdrasil
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Jilia Galway-Witham and Chris Stringer wrote a nice perspective piece on this issue in Science recently: http://science.sciencemag.org/content/360/6395/1296.full (Stringer was also an author on the Trends in Ecology and Evolution piece as well). The piece in Science discusses more about the intermixing on humans with hominids that migrated out of Africa before modern Homo sapiens such as the Neanderthals and Denisovans. The authors write:

With only a few dissenters, the strictest versions of both RAO (which denies interbreeding with other lineages or species) and multiregionalism (which argues for an interbreeding network of one species over the past ∼1.8 million years) are now generally regarded as falsified. Instead, two intermediate theories best accommodate the complex interactions between hominin taxa ∼40,000 to 100,000 years ago (8, 14): RAO with hybridization (RAOWH) and the assimilation model (AM) (see the figure).

The two theories differ in their reconstructions of the processes by which the DNA of dispersing H. sapiens populations mixed with those of other populations outside of Africa. AM emphasizes demic diffusion, in which populations of African-derived H. sapiens and Eurasian Neandertals and Denisovans would have mixed over wide areas. Genes would have flowed gradually between these regional populations, catalyzing genetic and anatomical changes and leading to the spread of modern traits. In contrast, RAOWH envisages H. sapiens genes as largely entering and traversing Eurasia within the bodies of dispersing humans of African origin. Along the way there were successful hybridization events with indigenous populations, but these were effectively absorbing fragmented populations of indigenes in a relatively rapid replacement process, where they overlapped. Debate continues over what constitutes necessary or sufficient evidence for either theory.
 
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