Timing of metazoan origins

A single cladistic analysis of Recent and Cambrian arthropods (see references Fortey, Briggs & Wills. 1997) has indicated cladogenesis events occuring back through the Precambrian, implying that the arthropods at least have a long and cryptic Precambrian history so far unrecorded in the fossil record. For example the millipede/insect group is placed near the base of the arthropod tree, below the point at which the crustaceans split from the chelicerates and trilobites. As this latter split has obviously already occured by the start of the Cambrian, this implies a long Precambrian history for the millipede/insect ancestors about which we know nothing.

Click on the thumbnail to look at a possible phylogenetic tree of Recent and Cambrian Arthropods. From Bioessays 19, pg 431. Fortey R.A., et al, 1997.

In a study published in 1996 (see references Wray et al. 1996) rates of gene sequence divergence for several phyla (chordates, echinoderms, arthropods, molluscs and annelids) were calculated, the sequences being calibrated against mean divergence rates from animals with known Phanerozoic fossil records. This is the 'molecular clock' technique. Mean divergence times between the four invertebrate phyla and the chordates were all deep within the Precambrian, an average of 1.0-1.2 billion years ago. However the molecular clock technique has been criticised, on the invalidity of applying divergence rates from vertebrate species to comparisons between chordates and other species, and because the 'clock' carries significant margins of error.

Both of these studies add to the growing evidence for metazoan life occuring further back in the Precambrian than previosly thought. They also cast doubt on the long held view of the Cambrian explosion being the divergence time for the majority of phyla. It now seems probable that phyla cladogenesis occured much earlier, possibly even as far back as 1 billion years ago.