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A New Aquatic Ape Theory

Wetlands and lake shores may have been important oases for early hominids, providing our ancestors with reliable food supplies and encouraging the evolution of upright walking

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A gorilla in the Congo wading in a swamp. Image courtesy of Wikicommons

The aquatic ape theory, now largely dismissed, tries to explain the origins of many of humankind’s unique traits. Popularized in the 1970s and 1980s by writer Elaine Morgan, the theory suggests that early hominids lived in water at least part of the time. This aquatic lifestyle supposedly accounts for our hairless bodies, which made us more streamlined for swimming and diving; our upright, two-legged walking, which made wading easier; and our layers of subcutaneous fat, which made us better insulated in water (think whale blubber). The theory even links an aquatic existence to the evolution of human speech.

The hypothesis was met with so much criticism that it’s not even mentioned in human evolution textbooks. But that doesn’t mean aquatic habitats didn’t play some kind of role in our ancestors’ lives.

In 2009, Richard Wrangham of Harvard University and colleagues suggested in the American Journal of Physical Anthropology (PDF) that shallow aquatic habitats allowed hominids to thrive in savannas, enabling our ancestors to move from tropical forests to open grasslands.

About 2.5 million to 1.4 million years ago, when the genus Homo emerged, Africa became drier. During certain seasons, already dry savannas became even more arid, making it difficult for hominids to find adequate food. But Wrangham’s team argues that even in this inhospitable environment there were oases: wetlands and lake shores. In these aquatic habitats, water lilies, cattails, herbs and other plants would have had edible, nutritious underground parts—roots and tubers—that would have been available year-round. These “fallback” foods would have gotten hominids through the lean times.

The researchers based their arguments on modern primate behavior. For example, baboons in Botswana’s Okavango Delta, which floods every summer, start eating a lot of water lily roots when fruit becomes scarce. And hunter-gatherers in parts of Africa and Australia also eat a lot of roots and tubers from aquatic plants.

The fossil record also hints at the importance of aquatic environments. Wrangham and his team looked at nearly 20 hominid fossil sites in East and South Africa. In East Africa, the geologic and fossil evidence suggests that hominids were living in areas with lakes or flooded grasslands. South African sites tended to be drier but were still located near streams.

The researchers say foraging in these environments may have led to habitual upright walking. Today, chimpanzees and gorillas occasionally venture into shallow bodies of water, and when they do, they wade on two legs. It makes sense. Wading bipedally allows the apes to keep their heads above water. As our earliest ancestors spent longer and longer periods of time wading upright, it became beneficial to evolve specialized anatomy for two-legged walking.

Wrangham and his colleagues acknowledge that their case rests on circumstantial evidence. There’s no direct proof that this is how hominids were living. And the evidence has alternative explanations. For instance, watery habitats allow for better fossil preservation, so finding hominids in watery locales may not be representative of where they actually spent most of their time.

So like most things in human evolution, the debate’s wide open. What role do you think wetlands and lake shores played in our ancestors’ lives?

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