Free The Next Species: The Future of Evolution in the Aftermath of Man by Michael Tennesen

on hominid intelligence. Trees and brush lined the river, though savanna grasslands dominated the greater landscape. Njau was born in Tanzania and got his BA there at the University of Dar Es Salaam and his PhD at Rutgers University before taking a position at Indiana University. He and Hlusko had worked together at other sites in northern Africa.
    We arrived on a cloudy day at the Grumeti River, where more than twenty hippos weighing 3,000 to 10,000 pounds (1,600 to 4,500 kilograms) glistened in the sun as they jostled with each other for a place in the water. Along the shores lurked four or five crocodiles, their rough, bumpy skin and long, toothy jaws blending eerily into the landscape. Though the hippos were not to be ignored, it was the crocodiles that drew the most attention from Njau.
    According to Njau, crocodiles are the most dangerous predators in Africa, causing far more deaths than lions or leopards. Crocodiles have killed more than five hundred people in Tanzania alone since 1985. Njau explained the inevitability of these occurrences: “Victims know where they are, how to avoid them, yet they still keep getting caught and killed.” Njau warned me that for every crocodile you see above the water, there can be several others below waiting. I stayed far back from the river.
    The reason for this caution was that crocodiles frequented the water where men or animals came to drink or bathe, and these secretive reptiles were very, very patient. At just the right time, when its prey, man or beast, came forward, convinced there was no danger, the crocodile would lunge out of the water with extraordinary speed and grab its victim. The crocodile locks its jaws on the head, shoulder, arms, or front legs of its prey, then drags its spoils back into the poolto be held down to drown.
    During his thesis study, Njau came to the Grumeti River in the early summer to observe how the crocodiles overtook other animals. He visited a few months later in the dry season when the river had vanished and the crocodiles and hippos had moved on. He studied the bones left in the middle of the pool where the river had dried out and compared the tooth marks left by crocodiles to those of other carnivores. He wanted to know what marks the different predators made so he could study fossils and better know what was happening back then.
    Crocodiles had rows of as many as sixty-six teeth along powerful jaws that were ideal for gripping prey. The crocodile would often grab a victim and beat it against a rock, or it would sometimes go into a death spiral and roll over and over, or two crocodiles would grab the victim and roll over in opposite directions. The crocodile tried to disarticulate a substantial chunk of meat and then swallow it whole, allowing the reptile’s stomach acids to do most of the digesting.
    Crocodiles left puncture wounds where their jaws locked on a victim. But they weren’t able to move their jaws from side to side. This meant they actually made fewer marks on the bones of prey than other predators, although in some instances they left dense concentrations of bites on bones they were unable to swallow. A crocodile would tend to rip a large piece of meat and bone from a victim, swallow it whole, and then leave the rest. Lions, leopards, and even hyenas would gnaw on the ends of the bones to get the meat off, and even break the bone to get at the marrow. Thus most of the bones ofcrocodile victims would show fewer tooth marks than the bones of lion or leopard victims, and lacked gnawing damage on the ends. Crocodiles would take only right-size chunks of meat—not too big to catch in its throat, but not too small not to warrant the effort of the hunt. Bones that didn’t fit either of these categories were left in the water to settle to the bottom of the pond, along with leftover crocodile teeth.

    Back at Olduvai on a day when the sun was beating hard, we revisited the high ridge of the gorge, parked our cars near a cliff, and