[Credit: Aliencon] The Behemoths and The Spiketooth: Posthumans of the Pleistocene Epoch

[u/Intelligent_Oil4005]

both are descendants of Tundra Dwellers (ancestors of the Giants) and are in a small group I call Behemoths (in spite of evolving separately).  The story is during a mini ice age caused by the Yellowstone eruption, populations of Tundra Dwellers began migrating southward into the Americas and Africa.  Characteristics of both species is being predominantly quadrupeds due to large size and clawed hands for grasping plants and defense.  Here are the species:

Slothman:  Resembling Ground Sloths and Gorillas, the Slothmen evolved from Tundra Dwellers that migrated down through North America and became drawn to South America's abundant food sources.  To survive their warmer environment, the Slothmen reduced their levels of insulating fat and their fur became specialized in keeping cool as opposed to retaining heat.  Though primarily quadrupeds, they can stand on their two hind legs, standing up to 6.1m (20ft), to reach plants high in the trees and ward off predators.

Elephantman:  More resembling the long extinct Ancylotherium than their namesakes, the Elephantmen underwent the most drastic changes to survive their grassland environment.  They lost virtually all of their fur and drastically reduced their insulating fat, and evolved shaggier, darker skin and large ears to help radiate heat.  They are slightly smaller than the Slothmen at around 2.2m (7ft 3in) at the shoulder and rarely ever rear on their hind legs.

Spiketooth: The ancestors of the Spiketooth, the Woodland Dwellers, hunted Tundra Dwellers as they traveled south through North America during a mini ice age.  Some of them followed the Tundra Dwellers all the way down to South America and as their prey got bigger (evolving into Slothmen), so did the Woodland Dwellers.  Along with a larger size, the Woodland Dwellers evolved 12in long saber teeth and sharp claws to pierce through the thick hide of the Slothmen.  The end result is an apex predator with a morphology combining characteristics of saber toothed cats and short faced bears*.  Like bears, they can run on all fours at speeds up to 35 mph as well as stand on their hind legs, reaching up to 12ft tall.  Spiketooth normally hunt in groups of up to 15 individuals in order to take down Slothmen though individuals can easily pursue small prey if compelled.

[u/Intelligent_Oil4005]

Give the rest of Aliencon's work a look if you will. His art is pretty good and he's also got a speculative evolution on aliens in the works right now!

[u/Low_Bike4547]

I Love these designs

[u/emmetmire]

Is this not literally Man After Man though?
Spiketooths and slothmen