The history of hominids provides strong evidence for the evolutionary separation of modern humans (genus Homo) from chimpanzees (genus Pan). This kind of divergence takes millions of years to occur successfully due to the rate of gene mutation and selection. Studies used to look at the average genetic variances between chimpanzees and humans. Now more accurate and with more attention to detail, newer studies go even further by looking at key common genetic sequences between chimps and humans and comparing their relative ages. Younger sequences suggest a four million year period necessary for the gradual separation of chimps and modern humans.
Hominid is the term used to describe members of the Hominidae family of humans. Hominidae includes all of those species on the human side of the family tree since the most recent common ancestor between humans and apes. For this reason, hominids are also sometimes referred to as “great apes.” The taxonomic classification of great apes has seen a fair amount of revision over recent years. The term Hominidae was originally used only to classify humans and close relatives of humans. Over the years, it has been broadened and generally accepted among scientists to include both humans and great apes.
Based on recorded physical evidence, it is commonly believed that the chimpanzee-human separation took place from six to eight million years ago. The molecular clock (a method used to calculate how long it takes for one thing to evolve into something else based on the rate of genetic mutation), however, suggests that the separation most likely occurred from 4.6 to 6.2 million years ago. Scientists strive to find more fossil evidence and to improve dating methods in an attempt to close the above mentioned gap in the timing and further understand the true details leading to the final separation.
Despite the popularity of the Pan/Homo separation belief, there is some supporting evidence for the minority notion that humans actually share a more closely related ancestor with orangutans (genus Pongo) than with chimps. This belief holds that the Homo/Pongo separation would have taken place as recently as only 13 million years back. It also more closely links the genus Pan with the genus Gorilla. Support for this theory lies in some characteristics that humans share with orangutans. Similar beard and mustache growth, dental structure and enamel thickness, a high level of estriol production, posterior palate thickness, shoulder blade arrangement, and a single incisive foramen are among the 28 (at the very least) characteristics which human uniquely share with orangutans. While humans and chimps share maybe a few (at best) such characteristics, it is still much more widely accepted that these physical similarities are misleading.
The most recent common Hominidae ancestor lived about 14 million years ago. At that time, ancestors of the orangutan family (genus Pongo) speciated from those of the other three extant genera of the Hominidae family, you may recall the discovery of the three to four million year old fossil that came to be known as “Lucy.” Lucy, a gracile australopithecine, had prominent snout-like features associated with orangutans. In July of 2001, a seven million year old fossil of a Sahelanthropus tchadensis skull was unearthed in Chad, Africa. This fossil, nicknamed Toumaï, had flattering facial features and may even be a direct ancestor of the humans of today. More fossil evidence that is similar must be found in order to finalize the direct or near-direct linkage.
The study of the human fossil record is called paleoanthropology. This term comes from area of study’s combined concentrations of both paleontology, which deals with ancient life forms comprehensively, and anthropology, which focuses solely on humans. The hominid fossil record is nowhere near completion. The evidence appears in fragmented stages, but the gaps continue to get filled in and a very informative outline of human evolutionary history continues to unfold. As more and more fossil remains are discovered and their common genetic links are observed, the case for evolution only grows clearer and more widely accepted. Scientists have gone from thinking that final separation occurred as many as 30 or 40 millions of years ago, to 15 to 20 million years ago, to as recently as between only five and ten million years ago.
Some of the species of apes that lived 15 million+ years ago, like the Ramapithecus, were once considered to be included in the hominid family and thought of as human ancestors. Upon finding more fossil evidence, Ramapithecus was found to be more closely linked with orangutans. It is precisely this type of conclusive evidence that paleoanthropology hopes to discover to complete the hominid fossil record. There are currently 21 hominid species according to the fossil record. Here they are in order of appearance in the record (not indicative of an evolutionary sequence):
Sahelanthropus tchadensis (named in 2002) – 6 or 7 million years old, the oldest known hominid species
Orrorin tugenensis (July 2001) – Has limbs that are 1.5 times larger than those of Lucy (about the size of a female chimp)
Ardipithecus ramidus (September 1994) – 4.4 million years old; a near-complete skull and partial skeleton of this species took 15 years to fully excavate, restore, and analyze
Australopithecus anamensis (August 1995) – 3.9 to 4.2 million years old, 21 fossils excavated in Kenya between 1988 and 1994, has a combination of primitive features (mostly in the skull) and more advanced features (found in the body)
Australopithecus afarensis – 3 to 3.9 million years old, bipedal, sexual dimorphism (the females were smaller than the males)
Kenyanthropus platyops (2001) – about 3.5 million years old
Australopithecus africanus – lived two to three million years ago
Australopithecus garhi (April 1999) – show a mixture of ape-like and human-like proportional qualities
Australopithecus sediba (2008) – hailed as a possible transition between A. africanus and Homo
Australopithecus aethiopicus – 2.3 to 2.6 million years old
Australopithecus robustus – 1.5 to 2 million years old, may have used bones as digging tools
Australopithecus boisei (formerly Zinjanthropus boisei) – 1.1 to 2.1 million years old
Homo habilis – 1.5 to 2.4 million years old, used tools, some controversy concerning its range of variation within a single species
Homo georgicus (2002) – about 1.8 million years old
Homo erectus – 1.8 million to 300,000 years old, possibly more efficient at walking upright than today’s humans
Homo ergaster – used by some to classify African erectus specimens with differentiating skull properties
Homo antecessor (1977) – 780,000+ years old
Homo sapiens – about 500,000 years old
Homo sapiens neanderthalensis – 30,000 to 230,000 years old, slightly larger brain size than modern humans, thick bones, powerful muscles, used tools and weapons
Homo floresiensis (2003) – found on an Indonesian island, thought to be a dwarf form evolved from Homo erectus (dwarf versions of larger mammals commonly exist on island civilizations)
Homo sapiens sapiens – about 195,000 years old, modern humans, extensive use of raw materials.