In the Western USA deserts ground sloths often held a desert (xeric) niche eating Joshua trees when other forage was not abundant. Joshua trees are very slow growing. The niche is not filled with the extinction of ground sloths. Tree eating herbivores have a strong tendency to grow larger over time (and thus thicker bones to support the weight), including in xeric environments. _________________________________________________________________________________________________Tree eating animal evolution can largely be seen in terms of linear evolution for larger body size. With extinction events followed by other often related populations entering the emptied niche and in turn growing larger over time. This is one of the rare cases since the Carboniferous when linear evolution is a good model. _________________________________________________________________________________________________In the last 2.1 million years and especially starting at about 600kya it is likely linear evolution is a good model for human evolution. Before 2.1mya linear evolution as in general is a poor model for human evolution and is one of the causes of a lot of incorrect thinking in paleoanthropology. Of course I am not the only messenger about linear evolution usually being a poor model, this is widely known in modern biology and more and more paleoanthropology (Fuentes etc.) are lecturing against the evils of linear evolution models and “missing links”.

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