The Savanna Theory
In the 90s a much loved theory about human evolution began to crumble quietly in the halls of science. Unlike when the fall of a scientific hypothesis is often caused by a better hypothesis, in this case there was only a vacuum to replace the Savanna theory. The issue in question was one of selective pressure; why and how humans evolved certain unique characteristics not shared by other primates.
- Why does homo sapiens walk on two legs?
- Why did he learn to speak?
- Why has he lost his fur?
This seldom questioned theory, though flimsy, has as its crux that man began to walk upright once he migrated from the trees to the open savanna. The height gained from bipedalism aided hunting, avoiding predators, with the side-effect being hands that loitered ineffectually at the side. Thankfully, instead of vestigial drumsticks like the T-Rex or a jump down the evolutionary ladder from homo sapiens to homo autoeroticus-constans, we instead picked up a freaking stone axe. We were the new badass stone age-lumberjacks on the scene, deal with it! Oh, and there was supposedly a bit of art and music somewhere in there too.
In a clear chicken or the egg scenario, the process of doing these new creative activities effected intelligence in the brain, which may have given rise to the ability to speak. But then, how did we do these new creative activities without the necessary intelligence in the first place? Let us pause for a second to scratch our collective chin. "Hmmm..."
As for the new "always naked" apes on the African scene, Savanna doesn't have much to offer in the way of an explanation. The larger problem with the Savanna theory is that it makes sweeping assumptions about humanity based upon a small subset of data from an isolated geography.
The whole "Head to grasslands, young ape-man" theory arose during a time when human fossils found in South Africa near grasslands were in the limelight. Anthropologists have of course made many other finds of even older humans living in wooded, wet areas bipedally. The famous Lucy was actually buried next to a lake.
A less well-known theory also exists that offers a different selective pressure as cause to bipedalism, speech, and hairlessness. Instead of the grasslands as trait incubator, humanity's ancestors go back instead to the water from which each mammal was born.
Tenets of the Aquatic Ape Hypothesis
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Every mammal that has become naked and hairless has an aquatic ancestor, even the Rhinoceros and Elephant. The one exception is the Naked Mole Rat, that lives its life entirely underground.
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Bipedalism. All primates have the ability to walk on two legs for a short time, but when they wade through the water they always walk upright. This phenomenon was noted by David Attenborough as well, a supporter of the Aquatic Ape hypothesis.
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Unlike all other primates, human have a subcutaneous fat layer (especially prominent in infants). Other aquatic mammals, such as the whale, possess this same type of fat layer.
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It is no shape in the way the throat is formed, or particular set of muscles in the tongue that the lesser primates possess that prevents speech. It is not even intelligence - the gorilla cannot even mimic the sounds of human speech. Why? The key difference lies in the ability to fine-tune the flow of fuel into the engine, breath through the vocal chords, that is unseen among non-human land mammals. The only creatures with this ability are diving animals and birds.
Of course, as with most discussions about the exact cause of a unique evolutionary trait gained hundreds of thousands to millions of years ago, the evidence does not prove causality. We have plenty of physical evidence to support something less unique, such as the return of some land-based mammals to the ocean. The unique traits of humanity, which remain the holy grail of evolutionary study, often are explained by very clever answers to "what if" questions.
Consider come counter-points to the AAH, courtesy of Wikipedia.
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Hairlessness - most aquatic mammals that are comparably sized to humans are not hairless, but have dense, insulating fur and swim very well, with fatty layers beneath the skin.
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Bipedalism – the disadvantages cited for bipedalism within the AAH are often the result of comparing humans to medium, terrestrial quadrupeds, but human evolution never included a period of quadrupedal locomotion.
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Body fat – the subcutaneous fat distribution in humans is more similar to a domesticated animal than an aquatic one, and is nearly identical to that of other primates.
But then, some counter-counter-points.
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Bipedalism out of water causes considerable problems for the back, knees and organs, while water would support the joints and torso and permit breathing.
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A hooded nose, muscular nostril aperture control and the philtrum preventing water from entering the nostrils.
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Vestigial webbing between the fingers
Clearly, in the battle between the Savanna and Aquatic Ape hypotheses there is no clear winner and loser. It may be that the questions that begin with Why... are so enchanting because their difficulty alone greatly increases the satisfaction that an answer brings. But of course, these issues hit so close to home that I secretly believe even the most staunch creationist feels a stirring of curiosity and wonder when news is brought to light. You know who you are, you crazy bastards! :D
The Scars of Evolution by Elaine Morgan
http://books.google.com/books?id=JkXvp0eAhAoC&printsec=frontcover
Elaine Morgan says we evolved from aquatic apes
http://www.ted.com/talks/lang/eng/elaine_morgan_says_we_evolved_from_aquatic_apes.html