Sunday, April 11, 2010

The evolution of bipedalism

on page 194-207, Jerry Coyne addresses the divergence of Homo sapiens from apes. The discovery of Lucy told scientists that "our upright posture evolved before our big brain" and "went against conventional wisdom that larger brains evolved first" (202). What anatomical modifications were necessary for bipedalism to evolve and why is it that modern apes are not physically able to walk upright? How did Johanson's discovery provide evidence that bipedalism evolved before our big brain? What might be the reason that bipedalism evolved first and what selective pressures/potential advantages may have caused such change? Finally, elaborate on the effects of bipedalism on human evolution - cultural ramifications, predation and foraging, migration and geographic distribution.


  1. Our place on the tree of evolution: Linnaeus proposed that we are in the order Primates because we share such traits with monkeys, apes, etc such as forward-facing eyes, fingernails, color vision, and opposable thumbs. But the missing link that provides the distinction between humans and chimpanzees has to do with the idea of bipedalism. Donald Johanson discovered Lucy: the first that has been discovered to have walked on two legs (that we can corroborate with the pieced-together fossil bones); the femur connects to the pelvis at one end and the knee at the other – the femurs angle in toward each other from the hips so that the center of gravity stay s in one place. Apes, on the other hand, are unable to walk upright because they knuckle-walk; their femurs are slightly splayed out causing them to be bowlegged (Coyne 200). With regards to analyzing a fossil: if the femurs angle toward the middle, it means the species is bipedal.
    Naming for the variations in brain size has been an ongoing debate, much like the debate on phylogeny/taxonomy and classification.
    The Australopiths were bipedal, had human-like hands and teeth but their brains were only about 1/3 the size of the brain of a present-day human (Campbell 34.8). Lucy had a brain size of that of a chimpanzee. Perhaps bipedalism evolved first because of climate and other environmental changes/pressures. An increased area of savanna with fewer trees caused selection to favor adaptations that made moving over open ground more accessible. But recently discovered fossils indicate that bipedaled hominins lived in mixed habitats ranging from forests to woodlands; and that the Austrapoliths had various locomotor styles. Therefore, reasons for the evolution of bipedalism may have been that plant nutrition was not fulfilling enough, the need to carry instruments and other material, and the need to care for the infant. These adaptations also lead the hominins to develop larger brains. (source: Overtime, humans adapted to using complex tools and cognitive transformation began to take place – DNA evidence links the gene FOXP2 to have played a key role in human language.

    Source: Campbell / Coyne

  2. When primates became bipedal there were a lot of anatomical changes that were necessary for the change. These anatomical changes produced very significant cultural changes as well that we don’t see in any other animals at all like primates. In the transition from quadruped to biped, the most heavily affected system in the body was the reproductive organs. First the female vagina moved forward to a point below the abdomen whereas on primates it is adjacent to the posterior end of the organism. As a result of this change in the vaginal cavity, the male penis changed as well. Over time, the penis that was best suited to reproduce was the one that could become most erect by shunting blood into the organ and constricting the veins. As a result of all these anatomical changes, human culture has changed in response. Primates tend to be polygamous and do not have any lasting bonds and scientists attribute this to the male approaching the female from behind and never making eye contact with the female during coitus. When humans were forced to face each other it created a whole new dynamic in relations and made us perhaps the only partially monogamous species in the world. Bipedal movement also created a male-chauvinistic society when compared to the gender roles in other societies because offspring could no long ride on the mother’s back but rather had to be held in the arms of the mother. Losing the function of one of her arms in holding the baby made the female more dependent on males for protection who never had to invest in the child care directly.
    Bipedalism is very advantageous in a Darwinian sense because it allowed us to walk on only two of our four limbs and use the other two in hunting, gathering, child care, etc. Thus tools were invented by the ability of our arms to not have to devote their time to walking. Imagine the possibilities that were caused when prehistoric bipedal hominids could use tools and hunt much more efficiently and greatly improve their chances of surviving and reproducing. Natural selection would heavily favor this adaptation. Since tools could range from anything like a rock to a bow and arrow, natural selection would favor larger brains that could wield these more complex tools more efficiently and greater improve odds of survival. Walking on two legs was the smartest thing ancient primates ever attempted.


  3. The ability to walk upright can be traced back to the anatomical bone structure of the organisms. The most amazing thing about the discovery of Lucy is that she, up until the discovery of Ardi, was the first nearly complete fossil that showed evidence of bipedalism. On page 200, Coyne explains how scientists can tell whether or not the organism is able to walk on two legs: "from the way that the femur connects to the pelvis at one end and to the knee at its other". In both modern Homo sapiens and in Australopithecus afarensis (Lucy), the thighbone is slanted inward while in modern chimpanzees, the thighbone is slanted away from the center line. With the femurs pointed inward, there is a greater ability for the organism to balance on its center of gravity, allowing it to walk upright. Modern chimps wobble with each step, requiring 4 limbs in order to balance. However, even though Lucy's pelvis and femurs resembled modern humans, her skull is closer to resembling modern chimpanzees. There are some modifications of the skull, such as a semiparabolic tooth row, but the skull cavity has a similar volume as the chimp. With this discovery, being that Lucy is closely related to the "missing link" between humans and chimps, scientists now believe that bipedalism evolved first.

    There are many theories on the reason for the evolution of bipedalism. The theory of thermoregulation suggests that standing up straight helps regulate body temperature by reducing the surface area of the body exposed to sunlight. A more popular theory deals with the organisms use of the other two limbs. It has been discovered that there are similar energy costs to bipedalism and quadrupedalism, but bipedalism allows the species free use of its hands. The use of hands has allowed hominids the ability of multitasking in regards to foraging and hunting. With foraging, the species could now pick fruit off of a tree and move to another tree without having to get on all for limbs to change locations. Similarly, especially after the development of weapons, the hands allow for hominids to attack its prey while chasing after it. The ability of multitasking has increased the specie's efficiency in gathering food and has become a selective advantage to their survival.

  4. Looking at the osteology of humans and their close relatives in the fossil record, there are a few key anatomical traits that are linked with (and allow for) bipedalism. One of these is the medially-angled femur. In a bipedally walking primate like ourselves, the femur is angled in toward the middle, just like it was in Lucy, proving that she could walk on two legs instead of walking on all fours. (Coyne 200). The angle “is created by the long neck of the femur in a human-like biped, which stabilizes the hip” (Bipedalism). Just like Mary stated, apes on the other hand, are unable to walk upright because they knuckle-walk; their femurs are slightly splayed out causing them to be bowlegged (Coyne 200). In addition, the distal end of the femur (called a medial condyle) of a human biped is considerably larger than of one in apes. This is an adaption that increases the stability of the knee joint and prevents the distal end of the femur from slipping out of alignment. Also, the curvature and "S" shape of the spines of humans and their closest bipedal relatives is another characteristic that helps distinguish a biped. Lastly, distinctly arched feet and non-opposable big toes helped humans diverge from apes and gain the ability to walk upright. When Mary Leakey and Andrew Hill discovered the “Laetoli footprints”in 1976 and examined the eighty-foot trail made by the two human ancestor hominins (Coyne 201), they soon realized that these footprints were “clearly those of a biped, showing an arch in the foot and a non-divergent big toe, since both of these structures help bear the weight of the body when walking” (Bipedalism). Modern apes don’t have all these adaptive traits and hence they are restrained to waddling on all fours.
    The two physical innovations which most obviously distinguish us from other primates are that we are bipedal and that we have bigger brains. Initially it was not known which of these differences appeared first; but this question was resolved with the discovery of the animals now called australopithecines: the name now given to the first bipedal primates. The australopithecines were unquestionably bipedal but their brains were no bigger than that of a chimpanzee. They had human-like hands with brains about 1/3 of the size of humans (Campbell 730). Lucy, discovered by Johanson, was exactly this scientific find that proved that bipedalism evolved prior to big brains.
    Just like Mintz said, bipedalism has myriad advantages in a Darwinian sense because movement on two limbs instead of on four allows bipedalists to partake in many more activities. These include the ability to transport food, feed in an upright, stationary position, avoid predatory attacks through better vision, better thermoregulation in tropical climates, and the ability to use tools in many different positions (Evidence for God). Survival and reproduction were definitely enhanced if all these traits were possible, and hence natural selection would heavily favor this adaptation. Favoring large brains that could wield these complex tools and engage in these complex mechanisms greater improved the chance of survival and therefore, human evolution towards bipedalism was extremely important. (Bipedalism) (Discover) (Evidence for God) (Pre-Human Bipeds)