Can we begin with the question of what caused humans to evolve? This requires knowledge of earth's history and dramatic land and climate changes. Specific to human evolution is the Pliocene period.
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NASA GISS: Research Features: Pliocene Global Warming: Page 2
The Pliocene epoch covers the period from approximately 5 to 1.8 million years ago and, as such, spanned the period of time during which the Earth transitioned from relatively warm climates to the generally cooler climates of the Pleistocene. This transition included the emergence of the direct ancestors of humankind and contains the beginnings of cyclic Northern Hemisphere glaciation. The Pliocene epoch itself contains episodic climate fluctuations prior to the late Pliocene cooling, and our focus for study is a warm period in the middle Pliocene between 3.15 and 2.85 million years before present.
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I think we need to go with the global warming during the Pliocene and the Aquatic Hyptheses. This means hairy primates got into the water to survive a heat wave. This pressured dramatic changes in the creatures that survived this period. We get naked apes, and the female naked apes have relatively large breast, ideal for a baby to suckle from when the mother and baby are in water. These naked apes are not completely naked, but have hair on their heads that grows longer than any other primate hair. This is great for a baby to grasp while parents and babies are in water.
Humans not only have less hair than other primates, but it is water streamlined, and these creatures have superior swimming ability, and their babies can swim when only a few weeks old. Like whales they have an insulating layer of fat, and unlike other animals they have sweat glands, and cry salty tears. Besides walking up right and having better use of their hands than any other primate.
In short humans evolved as they did, because of the period of time they spent in the water. The ones that survived and reproduced, were they ones that adopted to aquadic living best.