Bipedalism

Bipedalism in mammals

Most terrestrial mammals move naturally on all fours, over millions of years.
Mammals appear at the same time as the dinosaurs during the Triassic period, which ranges from -199.6 to -251,000,000 years.
The first primates (Purgatorius) appeared, there are about 70 million years. The first fossils, attributed to Orrorin tugenensis, the "first bipedal hominid", dating from 6 million years and show a fossil skeleton explicit about his ability to walk, with a femur very long and thick head. A set of fossils discovered in Kenya in 1995, we unveiled the first line of true bipeds, Australopithecus anamensis, which lived there about 4.5 million years. Homo habilis (-2.5 to -1.8 million years ago), Homo ergaster (-1.8 to 1 million years) and all hominid lineages until us, Homo sapiens (200,000 years) developed bipedalism. But man is not alone in using bipedalism as a means of locomotion.

Chimpanzees, bonobos, gorillas, capuchin monkeys, and orangutans, bears, suricates, kangaroos and many others regularly travel on their hind legs. All birds are bipeds like their ancestors, the dinosaurs, hominids, however, this ability has been selected to become in humans the only means of locomotion.
Already, there are 3.6 million years ago hominids were walking almost like men, recovered well on their legs. To analyze these traces (pictured opposite), the researchers compared the traces of human moving on the sand in different positions, posture upright to postures bent, knees bent like monkeys.
"... To our surprise, the Laetoli footprints fall completely within the normal range of modern human footprints. "David Raichlen (University of Arizona, Department of Anthropology).
But the findings of this study are not accepted by the entire scientific community.

Image: These precious footprints, attributed to the species Australopithecus afarensis, were "printed" in Tanzania, in volcanic ash, there are 3.6 million years. These traces are the oldest direct evidence of bipedalism in hominids.

Why hominids, they adopted bipedalism?

Why is man become bipedal?
This question haunts anthropologists and many theories have been advanced.
Fossils do not really tell us, that is why the answer to this question remains controversial.
The thesis of the savannah issued in 1925, is currently the most accepted within the scientific community. The man is standing on its hind legs to see above the tall grass and easily monitor their predators in the savannah.
The change of environment would be at the origin of this evolution, the Australopithecus have migrated from the forest to savanna. But this classical theory may be flawed because scientists have shown that desert existed long before the transition to savannah (semi-desert) and heavily forested areas persisted long after.

"While open habitats predominated for the creatures which some consider the oldest biped, biped newest, Australopithecus, evolved in forested habitats."
Craig Feibel, doctor of geology at the American University of Utah, he has conducted research in the environmental context of human evolution.
Hominids were no longer using quadrupedalism, or brachiation or arboreal suspension in its movements despite the presence of trees, but developed bipedalism.

Image: A female Suricate, watching from afar, the arrival of predators.
Meerkats are carnivorous mammals of the mongoose family, which live in southern Africa.

Why is man become bipedal?

The man would be restated to bring more food in his hands, a theory suggested by Gordon Hewes in 1961.
The transport of meat "considerable distances" would have been the key factor of bipedalism.
Observations on chimpanzees, by U.S. researchers, English, Japanese and Portuguese indicate that free hand could have been used mainly to carry food, quadrupedalism is extremely inefficient.
Described in Current Biology, scientists have developed tests in which guinea pigs were to pick up various food, drupes of African oil palm (Elaeis guineensis) fruit and hazelnut Africa (Coula edulis).
When the choice of food is great, chimpanzees adopt four times more bipedalism, in experiments where only palm fruit are offered.
To carry a maximum of fruits, a chimpanzee adopts bipedalism. Carvalho et al. In 2012.
But at the slightest alarm, they fled on all fours.
With them, quadrupedalism is more effective.

Another hypothesis very similar to that our ancestors would have gone to bipedalism to transport food, is one that argues that the man would be restated to free his hands to use tools.
Another theory was advanced in 2003 by the American zoologist Jonathan Kingdon's theory of power in a squatting position. A preadaptation to bipedalism would be mainly due to the ease of eating in a squatting position (collection of insects, worms or other). This painter and writer has mainly explained the importance of hands in hominid evolution.
Jonathan Kingdon was born in Tanzania and has lived and worked in East Africa.

Why hominids, they adopted bipedalism?

Carsten Niemitz, German anatomist, ethologist and evolutionary biologist, developed a theory of the evolution of human posture.
Hominids would have started walking upright, living along the shores. According to Professor bipedalism would have appeared in monkeys living on the edge of the water. The man was recovered along the shores where food was plentiful. There are a few million years in Africa, there were still huge lakes instead of deserts today. Based on studies on the loss of body heat from the primate tree, shape of bones, the well being of people at the water's edge and other evidence, he concluded that hominids have adapted to life at the water's edge. He gives example that their bodies would be adapted because heat loss in humans is mainly made ​​by the head and upper body, pelvis and legs are isolated by a layer of fat, as if the body had been in so to lose the least amount of heat while walking in water.
The monkeys rarely go into the water and heat is dissipated by the head but also by members who have little body fat.

This insulating layer may be an adaptation to walking in water. The food of tropical forests become rare by the time the man would have fallen back on consumption of fish caught in the ups and downs of lake water in climate change. Researchers searched for over thirty years in East Africa have long demanded why, near the remains of prehistoric man, we always find bones of animals that live in water, like crocodiles and hippos.
The discovery of Orrorin, considered until 2002 as the first hominid bipedal, indicated a stay in a wetland.
These indications, for the biologist Carsten Niemitz, who explained that men bipedal and standing was not born in the bush as is generally admitted, but the water's edge.

Image: Currently there are two primates showing a greater tendency than others to walk upright in the mangrove swamps of Borneo, the bonobo and proboscis, inhabiting a flooded forest.

Enigma of bipedalism

The enigma is far from settled because for 200 million years of terrestrial mammals moved on all fours with happiness. It is likely, there are 8 to 10 million years, that great apes and men separate. The man for 8 million years ago was forced to bipedalism, step by step, into a remarkable asset. The fragile knees, lumbar and femoral neck is the trace of this constant adaptation.
The evolution of man and all living beings from the beginning is conditioned by the environment and climate. Over the past million years the Earth's climate has oscillated between periods (100,000 years) of cold weather, interspersed with periods of warmer weather. These climatic variations gradually select the fauna and flora. To retrace the long journey of human evolution over millions of years, scientists have only fragmentary pieces of bone and teeth that have resisted time, a few teeth in the Tugen Hills in Kenya, some teeth in the Middle Awash in Ethiopia, half ape mandible on deposit Nakali in Kenya...
Despite little evidence, each discovery changes the scenario of evolution and back the date of divergence between apes and humans. In parallel to the Australopithecus lineage, develops the human lineage that moves better and better on his legs and gradually abandoned arboreal life. The bones of hominids relatively few make any hypothetical theories.

One can easily understand the difficulty scientists to rewrite the history of man, as the task is huge and impossible. It is as if we were to reconstruct the history of modern civilization in Europe, from a Gallo-Roman pottery and a lighter. That is why, all his theories are not very convincing. The theses of the savannah, the transport of food, life on the edge of the shore and the others are probably false. It is likely that this property human bipedalism, is not related to an event but to multiple factors that we will never know. But man is standing because it is a monkey and up and down trees or hanging monkey has a vertical station, so why seek an explanation for bipedalism other than trees. It was enough to hominid just down from the trees to be bipedal.
The Meerkats walk on their hind legs to see far, and monitor the arrival of predators and yet they have not adopted this means of locomotion. Chimpanzees temporarily adopt bipedalism to carry food or use a tool and yet they have not evolved into what one means of locomotion. Of course that man is an inhabitant of the shores, half of humanity lives on the edge of the water, water is vital to all living organisms.

Image: If our planet had no relief, liquid water would cover its entire surface to a depth of 3 km. Apparently, that's a lot of water, but the layer of 3 km is only 0.023% of the radius of the Earth. This is equivalent to the thickness of the skin of an apple compared to the apple. Image: NASA