The "innovation" most discussed is bipedalism. But unfortunately, nobody is going to prove if it was driven by behavior or genetics or anything else. Most apes can walk upright. Same goes for tool use. Both chimps and orangutans have been observed using objects as tools. However, as I stated, I don't think anyone has ever shown selection (i.e. fixation of genetic change) as the result of behavioral innovation. As a matter of fact, it is dang near impossible to even shown genetic correlation to behavioral innovation. You can show genetic links to some behavioral pathologies, and one could hold out some hope that said pathology might be selected for under certain conditions.
Bipedalism, language, tool use, social behavior, fire, + others all had effects. Unfortunately the fossil record is so sparse that making definitive claims about any of them is wishful thinking. In the case of fire, there is only very poor indications of early use, with better indications around 700-800Kya, and good indications of much wider use around 300Kya, which places fire near the end of much of the evolution which resulted in modern humans. Just my WAG.
For an example of how theories can proliferate in the absence of much data, the following are theories about the origin of bipedalism in humans (from wikipedia)
Humans
There are at least twelve distinct hypotheses as to how and why bipedalism evolved in humans, and also some debate as to when. Evidence points to bipedalism evolving before the expansion in human brain size. The different hypotheses are not necessarily mutually exclusive and a number of selective forces may have acted together to lead to human bipedalism.
Humans evolved from a chimpanzee-like, knuckle-walking ancestor. Various reasons have been proposed for the evolution of human bipedalism, including freeing the hands for tool use and carrying, sexual dimorphism in food gathering, changes in climate and habitat (from jungle to savanna) and to reduce the amount of skin exposed to the tropical sun. The first two explanations have been criticized for projecting modern social concerns and prejudices onto ancestral species. The latter two have been criticized for not making sense in the context of the forest and woodland biomes occupied by human ancestors. An alternative explanation is the mixture of savanna and scattered woods forced proto-humans to travel between clusters of trees and bipedalism offered greater efficiency for slow, long-distance travel between these clusters than knuckle-walking quadrupedism.[16]
[edit] Postural feeding hypothesis
The postural feeding hypothesis has been recently supported by Dr. Kevin Hunt, a professor at Indiana University. This theory asserts that chimpanzees were only bipedal when they ate. While on the ground, they would reach up for fruit hanging from small trees and while in trees, bipedalism was utilized by grabbing for an overhead branch. These bipedal movements may have evolved into regular habits because they were so convenient in obtaining food. Also, Hunt theorizes that these movements coevolved with chimpanzee arm-hanging, as this movement was very effective and efficient in harvesting food. When analyzing fossil anatomy, Australopithecus afarensis has very similar features of the hand and shoulder to the chimpanzee, which indicates hanging arms. Also, the Australopithecus hip and hind limb very clearly indicate bipedalism, but these fossils also indicate very inefficient locomotive movement when compared to humans. For this reason, Hunt argues that bipedalism evolved more as a terrestrial feeding posture than as a walking posture. As Hunt says, “A bipedal postural feeding adaptation may have been a preadaptation for the fully realized locomotor bipedalism apparent in Homo erectus.” A related hypothesis is that proto-humans learned upright posture not for picking fruit, as it is argued they would have stayed climbers if plucking fruit were all they were after, rather they learned to keep their head out of the water while searching for water plants, mollusca, and the like.
[edit] Provisioning model
One of the most elaborate theories on the origin of bipedalism is the behavioral model presented by C. Owen Lovejoy, which is known as "male provisioning".[17] Lovejoy theorizes that the evolution of bipedalism was a response to a monogamous society. As hominid males became monogamous, they would leave their families for the day in order to search for food. Once they found food for their family, the hominids would have to bring back the food, and the most effective way of doing this was through bipedalism.
There is some question over whether or not early hominids were monogamous. Some evidence indicates that early hominids, which were proven to be bipedal, were in fact polygamous. Among all monogamous primates, sexual dimorphism is greatly decreased. In Australopithecus afarensis, males were found to be nearly twice the weight of females (as well as a great deal taller), an attribute scientists would expect[citation needed] in a polygamous species. Lastly, monogamous primates are highly territorial, but fossil evidence indicates that Australopithecus afarensis lived in large groups. Overall, there is no concrete evidence to thoroughly discount or prove monogamy or polygamy in early hominids.
[edit] Other behavioural models
There are a variety of ideas which promote a specific change in behaviour as the key driver for the evolution of hominid bipedalism. For example, Wescott (1967) and later Jablonski & Chaplin (1993) suggest that bipedal threat displays could have been the transitional behaviour which led to some groups of apes beginning to adopt bipedal postures more often. Others (e.g. Dart 1925) have offered the idea that the need for more vigilance against predators could have provided the initial motivation. Dawkins (e.g. 2004) has argued that it could have begun as a kind of fashion that just caught on and then escalated through sexual selection. And it has even been suggested (e.g. Tanner 1981:165) that male phallic display could have been the initial incentive.
[edit] Thermoregulatory model
The thermoregulatory model explaining the origin of bipedalism is one of the simplest and most fanciful theories on the table, but it is a viable explanation. Dr. Peter Wheeler, a professor of evolutionary biology, proposes that bipedalism raises the amount of body surface area higher above the ground which results in a reduction in heat gain and helps heat dissipation. When a hominid is higher above the ground, the organism accesses more favorable wind speeds and temperatures. During heat seasons, greater wind flow results in a higher heat loss, which makes the organism more comfortable. Also, Wheeler explains that a vertical posture minimizes the direct exposure to the sun whereas quadrupedalism exposes more of the body to direct exposure.
[edit] Carrying models
Charles Darwin wrote that "Man could not have attained his present dominant position in the world without the use of his hands, which are so admirably adapted to the act of obedience of his will" Darwin (1871:52) and many models on bipedal origins are based on this line of thought. Gordon Hewes (1961) suggested that the carrying of meat "over considerable distances" (Hewes 1961:689) was the key factor. Isaac (1978) and Sinclair et al (1986) offered modifications of this idea as indeed did Lovejoy (1981) with his 'provisioning model' described above. Others, such as Nancy Tanner (1981) have suggested that infant carrying was key, whilst others have suggested stone tools and weapons drove the change.
[edit] Wading hypothesis
Main article: Aquatic ape hypothesis
The Aquatic ape hypothesis proposes that humans evolved bipedalism as a result of bipedal wading. Mammals that switch from quadrupedalism on land to bipedal wading appear mainly to be found among large primates, especially apes, with relatively few exceptions such as the grizzly bear. Bipedal wading has been observed in the bonobo, chimpanzee, lowland gorilla, orangutan, baboon and proboscis monkey. Bipedal wading provides the advantage of keeping the head above water for breathing.
[edit] Savannah hypothesis
This proposes that the onset of drier conditions severely reduced the amount of wooded habitats. During this period, when the forests became thin, early hominids adapted to an environment which was now more like the liminal forest-savanna mosaic zones of equatorial Africa. In order to remain effective in gathering food, the hominids had to travel relatively long distances with food or tools, thus making quadrupedalism extremely inefficient. Bipedalism developed both as an adaptation to facilitate movement across the grasslands and as a way to give early hominids use of their hands for food cultivation and tool use since they were no longer needed for locomotion. |
