Sorry to gnaw away at this, but one can't really found any strong arguments one way or the other on the alleged human lineage as the details are far too controversial. Whilst I'll concede that my notion of the time of H erectus's extinction was out of date, there is no agreement that those later so-called Homo erectus fragments have been correctly assigned. Neanderthal Man is commonly assigned to our own species, sharing as it does 99.5% of its DNA with Homo sapiens (and there's good recent DNA evidence of some interbreeding). Gould seems to be picking up on something that needn't be at all controversial. There's no evolutionary reason why three lineages of Homo couldn't have coexisted, and they certainly needn't have been derived one from the other any more than modern-day humans are derived from chimpanzees. That isn't how evolution works. As for his claim (not entirely correct as it happens) that the three lineages showed no evolutionary trends, several things. Their tenures on earth were all relatively short, and add to that the long generational span of hominid species and you have limited opportunity for evolutionary change. Then the fossil record is relatively poor. There is some evidence that different races of H. erectus developed in isolation. Lack of intermediates in the fossil record is a common issue in evolutionary biology and it has been addressed, not least, as I said, by Darwin himself. It's a good read.