The skull of Sahelanthropus tchadensis, widely considered the earliest fossil hominin following the evolutionary split between chimpanzee and human ancestors, was recovered from Chad in north-central Africa in 2001. A newly described femur (thigh bone) indicates, however, that this species was not adapted to walking on two legs as previously assumed. It likely walked on all four limbs or was arboreal, moving around in trees.

The shift to bipedal walking is widely considered the first evolutionary change in the human lineage. Researchers studying the femur propose either that Sahelanthropus was not an early hominin or perhaps the earliest hominins had not yet adapted to walking on two legs.  

Published in the Journal of Human Evolution December 2020 by Macchiarelli and colleagues.