A palm-sized skull fragment found in Kenya 27 years ago represents the earliest known direct human ancestor, or member of the genus Homo, according to a new analysis. The 2.4-million-year-old bone pushes the fossil record of human lineage back 500,000 years, assert anthropologist Andrew Hill of Yale University and his colleagues.
Their conclusion garnered mixed reactions from other investigators of human origins. One expresses reservations about the anatomical traits used to classify the fossil as a direct human ancestor.
The fossil fragment comes from the right side of the head and includes bone above the ear, the ear opening, the jaw joint and part of the skull base.
Identification and dating of the fossil provide the first solid evidence that early Homo fashioned the oldest known stone artifcats, which date to between 2.6 million and 2.4 million years ago at African sites, Hill's team argues in the Feb. 20 NATURE. The specimen also dates to a time when global temperatures rapidly cooled and many new animal species appeared. Yale geologist Elisabeth S. Vrba has theorized that this climate shift caused environmental changes that facilitated the emergence of the Homo genus.
Although Hill's group does not assign a species to the fossil, for lack of evidence, their report will influence the current debate over whether early Homo included only the traditionally accepted H. habilis or up to three separate species. "Our report makes it easier to conceive of a radiation of two or three Homo species by 1.8 million years ago," Hill contends.
The researchers compared the fossil with the skulls of African apes and hominids (members of the evolutionary family that includes modern humans), including species of the smaller-brained Australopithecus that lived from 3.7 million to 1 million years ago and the larger-brained succession of Homo species, H. habilis, H. erectus and H. sapiens. Two anatomical features identify the skull fragment as Homo, the scientists maintain. …