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But size alone does not explain our matchless reasoning skills. One of the mysteries of human evolution is that other species with large brains (such as Neandertals) seemingly did not achieve comparable levels of cognition. Could a cultural innovation, perhaps driven by rapid environmental changes, have contributed to the rise of symbolic thought, language and cooperative group society? Ian Tattersall speculates along these lines in "How We Came to Be Human," and William H. Calvin explores "The Emergence of Intelligence."

As our primate ancestors' intellects deepened, their bodies continued to morph. Their need to stoke the energy-consuming organ in their skulls with nutritious, calorie-rich fuel created selection pressure favoring features now characteristic of primates, such as grasping hands with opposable thumbs, relates Katharine Milton in "Diet and Primate Evolution." "To a great extent," concludes Milton, "we are truly what we eat." Other articles in the issue explore how and when early humans and our ape cousins began to sprawl around the planet. Even as recent discoveries answer some questions about our fascinating and complex history, they raise others. Alone among creatures alive today, we enjoy the ability to contemplate our species' odyssey through time. Food for thought.

Clues from Our Cousins

Planet of the Apes by David R. Begun
During the Miocene epoch, as many as 100 species of apes roamed throughout the Old World. New fossils suggest that the ones that gave rise to living great apes and humans evolved not in Africa but Eurasia.

Bonobo Sex and Society by Frans B. M. de Waal
The behavior of a close relative challenges assumptions about male supremacy in human evolution.

Diet and Primate Evolution by Katharine Milton
Many characteristics of modern primates, including our own species, derive from an early ancestor's practice of taking most of its food from the tropical canopy.

Why Are Some Animals So Smart? by Carel van Schaik
The unusual behavior of orangutans in a Sumatran swamp suggests a surprising answer.

The Human Odyssey

Stranger in a New Land by Kate Wong
Stunning finds in the Republic of Georgia upend long-standing ideas about the first hominids to journey out of Africa.

The Littlest Human by Kate Wong
A spectacular find in Indonesia reveals that a strikingly different hominid shared the earth with our kind in the not so distant past.

Founder Mutations by Dennis Drayna
A special class of genetic mutations that often cause human disease is enabling scientists to trace the migration and growth of specific human populations over thousands of years.

The Rise of Homo Sapiens

How We Came to Be Human by Ian Tattersall
The acquisition of language and the capacity for symbolic art may lie at the very heart of the extraordinary cognitive abilities that set us apart from the rest of creation.

The Morning of the Modern Mind by Kate Wong
Controversial discoveries suggest that the roots of our vaunted intellect run far deeper than is commonly believed.

The Emergence of Intelligence by William H. Calvin
Language, foresight and other hallmarks of intelligence are very likely connected through an underlying facility that plans rapid, novel movements.