Hominid Evolution and the Signor-Lipps Effect

November 27, 2005

While laid up recently, I read the Cox and Moore chapter about human evolution and impact on the Earth, and found it really fascinating. For one thing, it brought into focus just how briefly our species (as compared with our genus Homo) has been around.(about 115,000 years) and how rapid and substantial our impact has been.

Also while laid up, I finished reading James Powell's Night Comes to the Cretaceous (which I think was published in 1998). Among the various new insights goeologists have generated with the testing of the Alvarez's theory of impact as the major cause of dinosaur extinction, Powell included a passage about the Signor-Lipps effect (S-L) related to the fossil record (the effect is named for paleontologists Phil Signor and Jere Lipps who first identified it). S-L really relates to two effects:

  1. A sampling effect: the availability of rocks to be sampled can control the apparent abundance of fossils. This could lead researchers to conclude that some species went extinct before they really did.
  2. A boundary effect (derived from the sampling effect): the relative abundance of species in a geologic stratum may lead researchers to place a chronological boundary lower than it ought to be in interpreting a stratigraphic sequence. This should be avoided by cross-checking through all other available means to make sure that all species did not go extinct at approximately the same time. In other words, the relative abundance of species in a stratum should not be mistaken for a chronological time boundary.

Cox and Moore don't mention S-L with regard to the hominid fossil record, but it seems to me that researchers must be aware of and factor S-L into their interpretations of hominid evolution and dispersion. (Perhaps it has been, just not by Cox and Moore.) In particular, Figure 11.1 of Cox and Moore's book (page 398) shows a "scheme" for possible evolutionary relationships among the various hominid fossils. This figure strongly suggests that hominid paleontologists need a lot more information from the fossil record (and possibly comparative molecular evolution evidence as well) before humans will ever have a clear idea of our fossil and paleontologic lineage.

I bring up S-L to underscore the uncertainties inheret in hominid fossil scarcity. Often these early hominid fossils are found in fluvial deposits where an unfortunate individual hominid drowned (either by accident, by ancient foul play, or by natural flooding). To be sure, hominid fossil samples are tiny, as opposed to say ammonites or foraminifera, as well as relative to what probably was a relatively successful comtemporaneous hominid population.

A possible reason hominid paleontologists come up with so many hominid species (both of Homo and Australopithecus) is the variety of fluvial strata (both chronologically and spatially distributed) in which the fossils are found. There are of course substantial differences in dentition and bone detail (particularly as these pertain to cranial, pelvic, leg, and arm bones). Hominid paleontologists are constantly grappling with the question of just how much variation is enough to argue for a new species or genus in our heritage.This is made all the more difficult because hominids are known to have dwelled in savannah environments. If they were also forest dwellers, researchers would be hard-pressed to find fossil evidence of them, since the decay of organic matter occurs so rapidly in tropical forest such as those in equatorial Africa.