29.7B: Early Human Evolution
- List the evolved physical traits used to differentiate hominins from other hominoids
Human Evolution
The family Hominidae of order Primates includes chimpanzees and humans. Evidence from the fossil record and from a comparison of human and chimpanzee DNA suggests that humans and chimpanzees diverged from a common hominoid ancestor approximately 6 million years ago. Several species evolved from the evolutionary branch that includes humans, although our species is the only surviving member. The term hominin (or hominid) is used to refer to those species that evolved after this split of the primate line, thereby designating species that are more closely related to humans than to chimpanzees. Hominins, who were bipedal in comparison to the other hominoids who were primarily quadrupedal, includes those groups that probably gave rise to our species: Australopithecus africanus , Homo habilis , and Homo erectus , along with non- ancestral groups such as Australopithecus boisei . Determining the true lines of descent in hominins is difficult. In years past, when relatively few hominin fossils had been recovered, some scientists believed that considering them in order, from oldest to youngest, would demonstrate the course of evolution from early hominins to modern humans. In the past several years, however, many new fossils have been found. It is possible that there were often more than one species alive at any one time and that many of the fossils found (and species named) represent hominin species that died out and are not ancestral to modern humans. However, it is also possible that too many new species have been named.
Very Early Hominins
There have been three species of very early hominoids which have made news in the past few years. The oldest of these, Sahelanthropus tchadensis , has been dated to nearly seven million years ago. There is a single specimen of this genus, a skull that was a surface find in Chad. The fossil, informally called “Toumai,” is a mosaic of primitive and evolved characteristics. To date, it is unclear how this fossil fits with the picture given by molecular data. The line leading to modern humans and modern chimpanzees apparently bifurcated (divided into branches) about six million years ago. It is not thought at this time that this species was an ancestor of modern humans. It may not have been a hominin.
A second, younger species (around 5.7 million years ago), Orrorin tugenensis , is also a relatively-recent discovery, found in 2000. There are several specimens of Orrorin . It is not known whether Orrorin was a human ancestor, but this possibility has not been ruled out. Some features of Orrorin are more similar to those of modern humans than are the australopiths, although Orrorin is much older.
A third genus, Ardipithecus ramidus ( 4.4 million years ago), was discovered in the 1990s. The scientists who discovered the first fossil found that some other scientists did not believe the organism to be a biped (thus, it would not be considered a hominid). In the intervening years, several more specimens of Ardipithecus , including a new species, Ardipithecus kadabba (5.6 million years ago), demonstrated that they were bipedal. Again, the status of this genus as a human ancestor is uncertain, but, given that it was bipedal, it was a hominin.
Key Points
- Modern humans are classified as hominins, which also includes extinct bipedal human relatives, such as Australopithecus africanus , Homo habilis , and Homo erectus .
- Few very early (prior to 4 million years ago) hominin fossils have been found so determining the lines of hominin descent is extremely difficult.
- Within the last 20 years, three new genera of hominoids were discovered: Sahelanthropus tchadensis , Orrorin tugenensis, and Ardipithecus ramidus and kadabba , but their status in regards to human ancestry is somewhat uncertain.
Key Terms
- hominin : the evolutionary group that includes modern humans and now-extinct bipedal relatives
- hominoid : any great ape (such as humans) belonging to the superfamily Hominoidea