I found this information which was posted by Alicia Rich, who is a Biological Anthropologist and Primatologist, as it relates to why apes don't have tails and what might have been the driving factor in that change:
quote:First of all, great apes did not evolve on the African savannas and grasslands. Some people have argued that early hominins did, but that is a complicated debate in itself. In fact, there are only two neighboring communities of chimpanzees that are commonly referred to as "savanna chimpanzees," and just a couple more populations that we refer to as "dry habitat chimpanzees" (including the community that I study). When apes evolved during the Miocene, they likely split from their common ancestor with monkeys because of a change in ecological niche. Things were only slightly drier and more open in Africa than before (nothing like the dry, open place that it is today), so primates were still living in heavily forested habitats feeding in the trees. Today, most great apes still live in highly forested parts of Africa, and a few live in the drier "woodland" type habitats.
As their environment changed, and populations grew, there was probably a change in feeding ecology that pressured apes and monkeys to develop different traits to reduce competition. Monkeys today are able to feed on unripe fruit because of a different digestion system than ours. Apes are what we call "ripe-fruit specialists" (that includes us). Our digestive system cannot absorb enough nutrients to compensate for the digestive power needed for processing the unripe fruits. So as monkeys needed a physiology for moving around on the upper levels and inner portions of the trees where unripe fruits were abundant, miocene apes specializing on ripe fruits probably needed a different physiology for moving around on the peripheral branches and occasionally to the ground to collect the ripe fruits that reached maturity.
Once you picture a scenario like that, you should ask the question of why an individual might need a tail. For balance? Perhaps. Many monkeys with long tails are quadrupedal branch runners and walkers. Apes that do move in the trees a lot, tend to move below the branches instead of above. They hang by their arms to collect the ripe fruits on the peripheral branches, dropping down to the ground if the need arises. If the tail was no longer helping individuals to balance above branches, but instead was only there as another appendage to injure or waste energy developing, individuals with shorter and smaller tails would have been more efficient feeders, and better at surviving.
I'm no expert in this regard, but for those on the forum that are familiar with these fields of study, does the above explanation make sense?