Excerpt: 'The Link: Uncovering Our Earliest Ancestor'

In the center of the forest sits the lake, which is an endless source of fascination for the creatures living around it.

The crater it fills was formed by a volcanic eruption thousands of years before Ida was born. Deep underground, the earth's crust split, sending boiling-hot molten magma rushing toward the surface. Just before the magma broke through the surface, it collided with a layer of groundwater and instantly turned into steam, a process that eliminated all the lava. This reaction caused an explosion that blew out a large chunk of earth and left behind a vast crater a mile (1.6 kilometers) wide and more than eight hundred feet (250 meters) deep known as a maar lake.

Over time, the hole filled with a combination of groundwater seeping from below and rainwater trickling down from the heavens, and a lake was created. Though there are a few streams, no rivers flow in and out, so the water in the lake remains relatively still. Because of the lack of currents, the water at the bottom is cut off from the upper layers and unable to draw down oxygen from the atmosphere above. All the fish live near the top, and there are no scavengers prowling the lake floor.

The lake is so rich in algae that from above, it resembles a green eye in the middle of the rain forest. As the surface algae die, they sink to the bottom and turn to slime. Eventually the slime turns to mud. The combination of this heavy mud and an almost total lack of oxygen kills nearly all the bacteria, allowing any creature that perishes and sinks to the bottom to rest virtually undisturbed for eternity.

The lake is the heart of this forest ecosystem, and it sustains a diverse array of life. At the water's edge, enormous crocodiles patrol their territory, and frogs make a tock sound as they search for insects. The land frogs have short hind legs and dig for food, while the water frogs have long, thin legs. In the water, turtles with paddle-like feet push their way across the surface of the slimy water. Shells are attached to sunken rocks lining the sides of the lake. Repelled by the lack of oxygen at the deeper levels of the lake, the bowfin, perch, gar, and eels swim near the surface. Many of the fish eat hard-shelled prey such as snails.

As Ida moves through this vast array of wildlife, dodging bats and staying out of reach of the saw-toothed crocodile's snapping jaws, it's clear that she is different from the others resembling her. She's much lower in the trees than they are, and at first she appears to be playing with the other wildlife. But as she moves, it's apparent that she's tentative with her right wrist and her left arm.

Amid the tranquility of this Eden, and the chattering of hundreds of animals and the sounds of a squealing flock of bats, a rumble begins from beneath the lake and quickly becomes a roar, but the animals in the surrounding forest are oblivious to the large gas bubble erupting from deep within the earth's crust.

At that moment, Ida is leaning down for a drink from the lake, grasping the shaft of a palm tree with one hand and reaching to cup the water with the other. She seems apathetic about the disruption.

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