The age of the subfamily Homininae (of the
Homininae–Ponginae last common ancestor) is estimated at some 14 to 12.5 million years (
Sivapithecus). Its separation into Gorillini and Hominini (the "gorilla–human last common ancestor", GHLCA) is estimated to have occurred at about (TGHLCA) during the late
Miocene, close to the age of
Nakalipithecus nakayamai. There is evidence there was interbreeding of Gorillas and the Pan–Homo ancestors until right up to the Pan–Homo split.
Evolution of bipedalism Recent studies of
Ardipithecus ramidus (4.4 million years old) and
Orrorin tugenensis (6 million years old) suggest some degree of bipedalism.
Australopithecus and early
Paranthropus may have been
bipedal. Very early hominins such as
Ardipithecus ramidus may have possessed an arboreal type of bipedalism. The evolution of
bipedalism encouraged multiple changes among hominins especially when it came to
bipedalism in humans as they were now able to do many other things as they began to walk with their feet. These changes included the ability to now use their hands to create tools or carry things with their hands, the ability to travel longer distances at a faster speed, and the ability to hunt for food. According to researchers, humans were able to be bipedalists due to Darwin's Principle of
natural selection. Darwin himself believed that larger brains in humans made an upright gait necessary, but had no hypothesis for how the mechanism evolved. The first major theory attempting to directly explanation the origins of
bipedalism was the
Savannah hypothesis (Dart 1925.) This theory hypothesized that hominins became bipedalists due to the environment of the Savanna such as the tall grass and dry climate. This was later proven to be incorrect due to fossil records that showed that hominins were still climbing trees during this era. Anthropologist
Owen Lovejoy has suggested that
bipedalism was a result of
sexual dimorphism in efforts to help with the collecting of food. In his Male Provisioning Hypothesis introduced in 1981, lowered birth rates in early hominids increased pressure on males to provide for females and offspring. While females groomed and cared for their children with the family group, males ranged to seek food and returned bipadally with full arms. Males who could better provide for females in this model were more likely to mate and produce offspring. Anthropologist Yohannes Haile-Selassie, an expert on
Australopithecus anamensis, discusses the evidence that Australopithecus were one of the first hominins to evolve into obligate bipedalists. The remains of this subfamily are very important in the field of research as it presents possible information regarding how these primates adapted from tree life to terrestrial life. This was a huge adaptation as it encouraged many evolutionary changes within hominins including the ability to use their hand to make tools and gather food, as well as a larger brain development due to their change in diet.
Brain size evolution There has been a gradual increase in brain volume (
brain size) as the ancestors of modern humans progressed along the
timeline of human evolution, starting from about 600 cm3 in
Homo habilis up to 1500 cm3 in
Homo neanderthalensis. However, modern
Homo sapiens have a brain volume slightly smaller (1250 cm3) than Neanderthals, women have a brain slightly smaller than men and the Flores hominids (
Homo floresiensis), nicknamed hobbits, had a cranial capacity of about 380 cm3 (considered small for a chimpanzee), about a third of the
Homo erectus average. It is proposed that they evolved from
H. erectus as a case of insular dwarfism. In spite of their smaller brain, there is evidence that
H. floresiensis used fire and made stone tools at least as sophisticated as those of their proposed ancestors
H. erectus. In this case, it seems that for intelligence, the structure of the brain is more important than its size. The current size of the human brain is a big distinguishing factor that separates humans from other primates. Recent examination of the human brain shows that the brain of a human is about more than four times the size of
great apes and 20 times larger than the brain size of
old world monkeys. A study was conducted to help determine the evolution of the brain size within the sub family Homininae that tested the genes ASPM (
abnormal spindle-like microcephaly associated) and MCHP1
(microcephalin-1) and their association with the human brain. In this study researchers discovered that the increase in brain size is correlated to the increase of both ASP and MCPH1. MCPH1 is very polymorphic in humans compared to
gibbons,
Old World monkeys. This gene helps encouragegrowth of the brain. Further research indicated that the MCPH1 gene in humans could have also been an encouraging factor of population expansion. Other researchers have included that the diet was an encouraging factor to brain size as protein intake increased this helped brain development. The levels of
sexual dimorphism are generally seen as a marker of
sexual selection. Studies have suggested that the earliest hominins were dimorphic and that this lessened over the course of the evolution of the genus
Homo, correlating with humans becoming more monogamous, whereas gorillas, who live in harems, show a large degree of sexual dimorphism. Concealed (or "hidden") ovulation means that the phase of fertility is not detectable in women, whereas chimpanzees advertise ovulation via an obvious swelling of the genitals. Females can be partly aware of their ovulation along with their menstrual phases, but men are essentially unable to detect ovulation in women. Most primates have semi-concealed ovulation. Thus, one can think that the common ancestor had semi-concealed ovulation, which was inherited by gorillas, and that later evolved in concealed ovulation in humans and advertised ovulation in chimpanzees. Menopause also occurs in
rhesus monkeys, and possibly in chimpanzees, but does not in gorillas and is quite uncommon in other primates (and other mammal groups). ==See also==