Final answer:
Sahelanthropus tchadensis is one of the oldest species on the human family tree, dated to nearly seven million years ago. It has a mix of primitive and evolved features, and its exact role in human evolution is still under study. Other significant early human ancestors include the bipedal Australopithecus and the now-extinct Homo erectus.
Step-by-step explanation:
One of the oldest known species on the human family tree is Sahelanthropus tchadensis, discovered in Chad and dated to nearly seven million years ago. Although it has a combination of primitive and evolved characteristics, its position in the human family tree is still being debated by scientists. Notably, the species known as Australopithecus, which evolved about 3.9 million years ago, is recognized for being bipedal and is considered an early stepping stone towards modern humans.
The genus Homo, which includes our own species Homo sapiens, first appeared about 2.8 million years ago. Homo erectus is another significant species within this genus that has since gone extinct, known for its upright posture and smaller brain compared to modern humans. Various other species of the genus Homo have existed, but only Homo sapiens survive today.
The concept of hominin is used to describe species that evolved after the evolutionary split between humans and their closest relatives, chimpanzees. These species are more closely related to humans than chimps and are indispensable for understanding the evolution and characteristics that differentiate humans from other hominoids.