The final difficulty with this type of taxonomy is that it cannot be reconciled with our time scale. Simpson, Kurten, and others have shown that, within the geological periods with which we are concerned, a genus of mammals requires about eight million years to establish itself, and it usually makes no difference whether the animals are large or small, or fast or slow to mature.6
The oldest fossil-man remains that are definitely and indubitably Homo may be no more than 700,000 years old. If there really were, during the last 700,000 years, four genera of fossil men, including Homo, Pithecanthropus, Sinanthropus, and Atlanthropus, then these genera must have parted company early in the Pliocene, and we have neither manlike bones nor tools from this period.
Later on, after tools had appeared, we find that both Atlan-thropus in North Africa and Homo in Europe were making stylistically similar stone implements. Although a great many claims can be made for parallel evolution, it is inconceivable that men of two distinct genera could have made similar tools. The concept that the fossil men so far found, who lived during the last half million years, belonged to more than one genus is impossible both anatomically and in terms of behavior, as revealed by archaeology. This concept must be abandoned, and indeed many zoologists and anthropologists have already discarded it. Of the names proposed for our genus, Homo has two centuries of priority, and Homo is what we are, what our known ancestors were, and what our unknown ancestors could have been for as long as eight million years.