KOOBI FORA, Kenya — About 1.5 million years ago, individuals of two different species in the human evolutionary lineage trudged on a muddy lakeshore in northern Kenya, leaving behind intersecting trackways alongside the footprints of antelopes, horses, warthogs, giant storks and other animals.

These tracks turned into fossils that scientists have now discovered at a location called Koobi Fora, providing the first evidence that these two species — Paranthropus boisei and Homo erectus — shared the same landscape, literally crossing paths. The discovery raises intriguing questions about the relationship between the two species and any competition for resources.

Paranthropus boisei, the more distantly related to modern humans of the two, lived from about 2.3 to 1.2 million years ago, standing up to about 137cm tall. They had a skull adapted for large chewing muscles, including a cranial crest like those in male gorillas, as well as massive molars. Their feet bore ape-like traits including in the big toe.

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