Chris at Highly Allochthonous has an interesting post on recently discovered tracks made on a sandy sea floor by a giant protist. Similar fossil tracks have been found in early mid Proterozoic sediments. Chris gives an example from Australia (2 billion years old) but there are reports of tracks and even interpreted burrows from mid Proterozoic Vindhyan basin (1.5 -1.8 billion years old) of central India. These have been interpreted by some researchers to point to a deeper origin of metazoans (multicellular animals) than molecular phylogeny and the bulk of the body fossil record support. These data suggest a timeline of around 600-700 million years ago for the origin of metazoans.
The tracks made by the unicellular protist means that such deep origin interpretations are again in doubt. I've been reading claims and counterclaims on this issue for quite a while. I would like to add to what Chris has discussed by giving a list of the reasons why most paleobiologists feel that the pattern of appearance of macroscopic animal fossils faithfully records evolution. In effect the list below is a critique of the view that animals evolved much much before what the macroscopic fossil record indicates. I have taken these from an essay by Simon Conway Morris, an expert in early animal evolution.
1) these ancient “traces” are typically highly restricted in distribution and sometimes only known from a single slab of rock.
2) no explanation is offered as to the failure of the “organisms” to diversify in what would otherwise appear to be an ecological “vacuum”.
3) the structures in question are almost all remarkably different from one another.
4) all other evidence points to a “microbial world” uneffected by a putative macroscopic ecology ( i guess by this he means that the preservation of bacterial mats as stromatolites suggest a minimal disturbance of the sea floor by large critters).
5) the important fact that even in late Neoproterozoic sediments yielding exquisitely preserved softbodied algae, there is no corresponding preservation of metazoans.
So there are very good reasons to be skeptical of claims that diverge widely from evidence given by molecular phylogeny and the body fossil record.
There is a misconception I have encountered that for several billion years "nothing much" of evolutionary importance took place, that the planet was inhabited by boring bacteria and then unicellular eukaryotes. Then suddenly in the early Cambrian macroscopic life took hold. Such a view prevails because people focus on the evolution of form. But we do know that we share with protists many of the molecular mechanisms responsible for cell physiology and even more complicated processes like gene shuffling and meiosis are present at various levels of development in protists. All these must have been evolving for hundreds of millions of years before the origin of metazoans.
Multicellular animals evolved from protist ancestors. They were built on this already existing molecular scaffolding.
The origin of metazoans can be seen in this light as evolution favoring an increase in size made possible by ecological triggers perhaps like an increase in the level of atmospheric oxygen by the latest Proterozoic. The novelty was the evolution of development. Creatures grew larger by cells dividing into specialized groups and sticking to each other for a large part of the life cycle of the organism.
Even this did not appear in a jiffy. The Cambrian "explosion" is an awful and nebulous term that confuses more than clarifies. We now have enough of a fossil record spanning the latest Proterozoic -Mid Cambrian - spanning nearly 60 million years- that indicates the evolution of successive grades of animal complexity in several bursts of diversification. The early-mid Cambrian phase (the "explosion") represented by Chenjiang- Burgess type fauna was the youngest of such events in which triploblastic animals rapidly (geologically speaking) diversified. The figure below summarizes our present state of knowledge.
Source: Origin of Phyla
Complex multicellular animals evolved from slightly less complex ancestors which in turn....