Haikouella Seen here with different fossils on both the obverse and reverse, it is a very unique specimen from the Chengjiang Biota. One one side is a mass mortality of Haikouella lanceolata, thought by its describers to be the earliest craniate-like chordate. This fish-like animal has many similarities to the contemporaneous Yunnanozoon lividum, but differs in several aspects: it has a discernible heart, dorsal and ventral aorta, gill filaments, and a neural chord. For all these reasons, Chen, Huang, and Li identified it in the seminal Nature paper as a chordate. The debate rages on, but whatever the outcome, this creature was much like the ancestor of all the vertebrates. It derives its generic name from its resemblance to the modern day lancet Amphioxus. Such assemblages have been mentioned in the literature, and must represent some catastrophe that overcame them in mass. Xidazoon The reverse holds an unusual fossil that is a rare member of the Vetulicolians, an enigmatic group which some scientists place in their own phylum (Phylum Vetulicolia). They are thought to have been swimmers that either were filter feeders or detritivores. One researcher places them with the Urochordates, giving them strong affinity with the Phylum Chordata. At present, there is no agreement as to their systematic placement. This one is quite unusual with a circular mouth which was thought to have served it in filter-feeding.

Most major animal groups appear for the first time in the fossil record some 545 million years ago on the geological time scale in a relatively short period of time known as the Cambrian explosion. While some scientists believe there was indeed an explosion of diversity, others believe that such rapid acceleration of evolution is not possible; they posit that there was an extended period of evolutionary progression of all the animal groups, the evidence for which is lost in a sparsely populated fossil record.

The theory of the Cambrian Explosion holds that, beginning some 545 million years ago, an explosion of diversity led to the appearance over a relatively short period of 5 million to 10 million years of a huge number of complex, multi-celled organisms. Moreover, this burst of animal forms led to most of the major animal groups we know today, that is, every extant Phylum. It is also postulated that many forms that would rightfully deserve the rank of Phylum both appeared in the Cambrian only to rapidly disappear. Natural selection in many cases favored larger size, for example, hard skeletons to provide structural support - hence, the Cambrian gave rise to the first shelly animals and animals with exoskeletons (e.g., the trilobites), and the size of many animals also "exploded".

By the start of the Cambrian, a large supercontinent comprising all land on Earth was breaking up into smaller land masses. This increased the area of continental shelf produced shallow seas, and an expanded diversity of environments in which animals could specialize and speciate. The debate persists today about whether the evolutionary "explosion" of the Cambrian was as sudden and spontaneous as it appears in the fossil record.

The discovery of new pre-Cambrian and Cambrian fossils help, as these transitional forms support the hypothesis that diversification was well underway before the Cambrian began. More recently, the sequencing of the genomes of thousands of life forms is revealing just how many and what genes and the proteins they encode have been conserved from the Precambrian. The explosion of external form in the fossil record is what we see, but more gradual adaptation was taking place at the molecular level.

Wang et. al. (1999) for example, recently conducted phylogenetic studies divergences among animal phyla, plants, animals and fungi. These researchers estimated Arthropods diverged from more primitive chordates more than 900 million years ago, and Nematodes from that lineage almost 1200 million years ago. They furthermore estimated that the plant, animal and fungi Kingdoms might have split almost 1600 million years ago. Finally, they conjecture that the basal animal phyla (Porifera, Cnidaria, Ctenophora) diverged between about 1200 and 1500 million years ago. If their research is valid, at least six major metazoan phyla appeared deep in the Precambrian, hundreds of millions of years before the oldest fossils in the fossil record.

A facinating aspect of the Cambrian explosion is its speed over some 10 million years. From this it is reasonable to infer that expanded genomic complexity occured much earlier, perhaps over a billion years, prior to the morphological (phenotype) diversity that appeared in the Lower Cambrian.

In recent years, research has shown that genomic complexity "happens" in many ways, including duplication and deletion of genes, cascades of genes, and, in complex organisms, entire chromosomes can be affected. Interesting also, is that such geneomic scrambling is an important mechanism in the etiology of cancers in animals. Genomic diversity is, of course, the stuff on which natural selection operates. The more numerous and complex enviroments and ecosystems provided the varying selective pressures to amplify benefitial mutation (genotypes) within populations, prune detrimental mutations, and otherwise fine-tuning genomes to enhance survival. Such fine-tuning would be different in different ecological niches.

Among the famous Lagerstatten from Cambrian time, the Burgess Shale of Canada and Chengjiang, in Yunnan Province, China are the best known, having a great diversity of benthic or burrowing creatures, many of which are soft-bodied, lacking an exoskeleton. Less well known is that the American state of Utah where similar Cambrian creatures are found. If fact, some researchers believe a larger number of species are to be found in the Wheeler and Marjum Formations of Utah than in the Burgess Shale, though the fossils in Utah are far less abundant.

SLAB CONTAINING SPECIMENS of Pteridinium from Namibia shows a prominent organism from the earth's first multicellular fauna, called Ediacaran, which appeared some 600 million years ago. The Ediacaran animals died out before the Cambrian explosion of modern life. These thin, quilted, sheetlike organisms may be ancestral to some modern forms but may also represent a separate and ultimately failed experiment in multicellular life. The history of life tends to move in quick and quirky episodes, rather than by gradual improvement. To understand the events and generalities of life's pathway, we must go beyond principles of evolutionary theory to a paleontological examination of the contingent pattern of life's history on our planet - the single actualized version among millions of plausible alternatives that happened not to occur. Such a view of life's history is highly contrary both to conventional deterministic models of Western science and to the deepest social traditions and psychological hopes of Western culture for a history culminating in humans as life's highest expression and intended planetary steward.

It is important to remember that geological history contains numerous periods of slow evolution punctuated by periods of rapid evolution, which Steven J. Gould called Punctuated Equilibrium. The rates of evolution generally depend on rates of selection, which in turn depend on rates of environmental change. It also depends upon the existing genomic diversity on which selection acts. Mutation rates tend to be slow and steady, and in the absence of environmental change, slowly accumulate in a population. It is selective pressure that weeds out the mutations that are detrimental or neutral to survival, and retains and multiplies the mutations that are beneficial within a population. For a population isolated in a new environment, rapid selection can lead to speciation, and in the Lower Cambrian, to radically new forms that we now group in the Phyla of modern times.

The years ahead should see furtherance of knowledge of how and the timeline along which the Tree of Life branched, especially when proteomes of its many branches are unraveled. Still, major mysteries are likely to persist, given the amazing ability of nature to splice, dice, reassemble, swap, amplify, and silence or re-use nucleid acid sequences within the genome of living organisms.