Cambrian explosion

The Cambrian explosion refers to the geologically sudden appearance of complex multi-cellular macroscopic organisms between roughly 542 and 530 million years ago ( mya). This period marks a sharp transition in the fossil record with the appearance of the earliest members of many phyla of metazoans (multicellular animals). The "explosive" appearance of this adaptive radiation results both from rapid evolutionary change and the limits of previous technology to appreciate microfossils which formed the foundation of the fossil record before this time.

From the modern point of view, the apparently explosive radiation from obscure beginnings was partly an artifact of disregarding microfossils, which were scarcely detectable with 19th-century technology, and concentrating solely on the hard-shelled macrofossils that defined the phyla well established by 19th-century biologists, all of which were multiple-celled metazoa. Apparently abruptly, many kinds of fossils appearing in the Burgess Shale were seen showing obvious skeletal body features, whereas the traces of the hard-to-analyze " small shelly fauna" of Cambrian beginnings were ignored.

With time, advanced microscopy has gradually revealed the range of earlier microfossils. Prior to the discovery in 1909 of the Burgess Shale—incompletely published at the time and largely forced into existing categories as "precursors"— no fossilizations of early soft-bodied organisms had been published, and the vast reach of undiscovered earlier life was consigned to an enormous space of time— the " Pre-Cambrian" of old-fashioned schoolbooks.

More recent microfossil finds have shown "Pre-Cambrian" life consisting of more than single- celled organisms or simple diploblastic fauna. In 1994, phosphatized triploblastic embryos were discovered in rocks from southern China (Xiao et al. 1998). Evidence for Ediacaran triploblasts was available long before this discovery.

Fossils

This period of evolution is source to some of the most unusual fossils ever recovered. A single formation, the Burgess shale, has provided some of the best insights into this period of dramatic evolutionary change and experimentation that laid the foundation for most major modern animal body plans. Also appearing at this time are a wide variety of enigmatic and exotic configurations that appear to be unrelated to any modern animals. Fossils from the Chengjiang biota, about 10 million years older than the Burgess Shale, have given both a broader and sharper view of early Cambrian life.

Before the explosion, the fossil record is dominated by single- celled organisms with only the rare soft-bodied Ediacaran fauna and certain microfossils showing that multi-cellular life forms had arisen roughly 30 million years earlier (Xiao et al. 1998).

With the Cambrian explosion came the evolution of shells and other hard body parts. As shells are more easily preserved in sediment than soft body parts, this makes life forms of this and subsequent periods much easier to study in the fossil record than their Precambrian counterparts. This also contributes to the perception of an abrupt change in the fossil record.

Causes of the Cambrian explosion

The Cambrian explosion may have been precipitated by several environmental changes occurring in and just before this period. First the Varangian glaciation gave rise to a Snowball Earth in which all, or nearly all, of the oceans are covered entirely with ice. This was followed by a deglaciation and rapid global warming just before the beginning of the explosion itself.

In modern Arctic environments, single-celled organisms often form mats on the underside of ice sheets in order to maximize their exposure to sunlight. It is possible that adaptations useful to the maintenance of such colonies also assisted in the formation of the first triploblastic animals estimated to be 570 million years of age (Xiao et al. 1998). In addition, the Snowball Earth environment would have given rise to relatively few ecological niches, so the subsequent deglaciation and global warming may have provided an impetus for rapid evolution to fill many new environments.

Diversification

Of the 20 metazoan phyla with extensive fossil records, at least 11 first appeared in the Cambrian. Of the remainder, 1 is known to Precambrian and the other 8 first appear more recently (Collins 1994). An additional 12 soft-bodied phyla have poorly defined fossil records, but it is speculated that a significant number of these may also be Cambrian in origin.

Though this period is definitely of special significance in terms of rapid diversification and the emergence of new forms, some of that significance is likely to be overstated by the focus on macroscopic forms in the ways phyla are observed and defined. Molecular evidence suggests that at least six animal phyla had established themselves as distinct evolutionary paths during the Precambrian (Wang et al. 1999).

The sheer variety of forms found in the Burgess shale and other sites, has made some skeptical that single period of ~10-15 million years could have been long enough to give rise to such diversity. An emerging view is that the Cambrian explosion is the macroscopic conclusion to a prolonged period of evolution begun ~30 million years earlier with the innovation of multi-cellular organisms.

The Cambrian "explosion" continues to fascinate the non-specialist. Stephen Jay Gould's Wonderful Life (1989) provides the best general account for the layman of the paleontological analysis of the Burgess Shale; the book was the 1991 winner of The Aventis Prizes for Science Books.