The first humans (true hominims) after the human-chimp splint (YouTube):
Oldest to youngest:
The earliest potential hominin is Sahelanthropus tchadensis, discovered in Chad and dated to between 7 and 6 million years ago. While
earlier analyses of its skull suggested bipedalism, more recent studies
of postcranial bones (like the femur) propose it was not adapted to
walking on two legs, leading to debate about whether it was a true
hominin or if bipedalism developed later in the human lineage.
Orrorin
tugenensis, meaning "original man," is a fossil species from around 6
million years ago, discovered in Kenya in 2000, that is considered one
of the earliest possible hominins (human ancestors). The
species' defining feature is its femur (thigh bone), which shows
adaptations for bipedalism (upright walking), suggesting that upright
walking may have evolved very early in the hominin lineage. However,
the precise phylogenetic position of Orrorin is still debated, with
some researchers placing it as an early ancestor to modern humans and
others suggesting it might have been an evolutionary offshoot or a
relative of the robust australopithecines.
The Ardipithecus genus,
containing the species Ardipithecus kadabba and Ardipithecus ramidus,
represents a group of early hominins from Africa that lived between approximately 5.7 and 4.4 million years ago. First found in Ethiopia. While
not necessarily a direct human ancestor, Ardipithecus is important for
its intermediate traits, such as a mosaic of features for both upright
walking and climbing in wooded environments, which offer insights into
human evolution after the split from our chimpanzee ancestors.
Australopithecus was a
diverse group of extinct, ape-like hominins living in Africa between 4.4
and 1.4 million years ago, known for bipedal (upright) walking and
large molar teeth but with relatively small brains. First found in South Africa. These "southern
apes" are crucial to human evolution, as they were the ancestors of the
Homo genus, though the specific ancestral species is debated. Key species include the famous "Lucy" (Australopithecus afarensis), whose fossils confirmed that bipedalism predated the evolution of large brains.
Kenyanthropus is a genus of early human ancestors that lived in Africa about 3.5 million years ago, contemporary with Australopithecus afarensis ("Lucy's species"). Discovered in 1999, Kenyanthropus platyops
is known for its unusually flat face and small molars, traits that
distinguish it from Australopithecus and suggest greater diversity in
the human family tree at that time. While
some researchers view Kenyanthropus as a distinct genus, others argue
it represents a Kenyan variant of A. afarensis, with its unique features
potentially linked to the distorted nature of the fossil evidence.
**********************
So bipedalism; then large brains; then, of course, tool-making, and language.