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 –
