Final answer:
Hominids evolved from having smaller brains and larger jaws to resembling modern humans, as observed in fossil records from Australopithecus afarensis to Homo habilis and Homo naledi. The transition to bipedalism and characteristics like increased brain size and opposable thumbs marked the evolution. Multiple hominin species existed at the same time, with not all being direct ancestors of humans.
Step-by-step explanation:
Evolution of Hominid Appearances
The appearance of hominids has evolved significantly from the oldest to the youngest fossil records. The fossil hominids are arranged chronologically, showing that early forms had smaller brains and larger teeth with a more prominent jaw, as seen in Australopithecus afarensis. Over time, features became more similar to modern humans, with Homo habilis showing a reduction of jaw prognathism and an increase in brain size, around 600 to 750 cubic centimeters. Yet, Homo habilis retained some primitive features like elongated arms. Moreover, as hominins evolved, they adapted to upright walking, with Homo naledi having feet virtually indistinguishable from those of modern humans.
Understanding hominin evolution has been challenging, particularly with new fossil discoveries revealing that multiple hominin species existed concurrently. Some of these species were not directly ancestral to humans but were rather offshoots of the hominin evolutionary tree. Overall, human evolution has been marked by a shift towards bipedalism, increased brain size, and the development of fully opposable thumbs.