Fossilized bacterial colonies provide some of the first evidence of life on earth.
The Precambrian “era” is an overarching span of roughly four billion years from the formation of the Earth up to the beginning of the Cambrian era 542 million years ago. Fossil deposits from the Precambrian are relatively sparse compared with later periods due to the small size and soft, poorly fossilized bodies of Precambrian life. The fossils that paleontologists find invariably belong to aquatic organisms. There are numerous Precambrian deposits across the globe that provide valuable insights into this earliest era of life on Earth.
Living stromatolite colonies are rare but can be found in Shark Bay, Australia and Yellowstone National Park.
Microfossils of the first bacteria are estimated to be 3.5 billion years old–an impressive feat, considering that the first solid rocks are just 3.8 million years old. The remains of microorganisms similar to today’s bacteria are found in chert, a type of ancient silica-rich rock deposit found in parts of Ontario and Minnesota.
Evidence for bacterial life en masse is available in the form of stromatolites, rounded rock-like accretions formed when filmy bacterial colonies grow atop their forebears. Stromatolites are common in Precambrian fossil deposits and can occasionally be resolved down to the level of individual fossilized bacteria. Stromatolites were supplanted by more advanced microbes around one billion years ago.
Tracks and Burrows
Some of the fossilized burrows from the Precambrian are undoubtedly the traces of soft-bodied worm-like organisms.
The most direct evidence of active animals in the Ediacaran period of the late Precambrian is the tracks and burrows they left behind. As of 2011, it is unknown which of these fossilized tracks are formed by early metazoans (multicellular animals) since abnormally large single-celled organisms probably also inhabited the Precambrian oceans. Thus, all that we can say about these tracks–some of which are as old as 1.8 billion years–is that they were probably created by moving macroscopic creatures.
Some fossils from the Ediacaran period are of uncertain origin–they may be animal, fungal or some form of organism unknown to current biology.
Precambrian sites such as the Avalonian fossil deposits in Newfoundland frequently contain anomalous shapes that are almost certainly the remains of multicellular organisms. These fossils appear to be the remains of primitive animals with radial or bilateral symmetry, some of which apparently left tracks in the sea floor. The appearance of these fossils coincides with an increase in atmospheric oxygen in the late Precambrian.
The exact nature of these creatures is as yet unknown; track remnants indicate that they may have moved, and some of the organisms (such as the bilateral Dickinsonia) may have been the precursors to modern phyla. Due to the paucity of the fossil record, it is difficult to determine the nature of these organisms; it is uncertain whether they were animals at all. All the fossils tell us for sure is that these organisms had complex organization and distinct symmetry.
Poriferans resembling this sea sponge were the precursors to all animal life on Earth.
Between the microfossils found in stromatolites and chert and the myriad of animal forms in the Cambrian era was a period of simple, soft-bodied Precambrian fauna. This is poorly represented in the fossil record due to the soft bodies of these organisms. Nevertheless, fossils of primitive organisms similar to today’s cnidarians (jellyfish) and poriferans (sea sponges) are unambiguously present by 580 million years ago (38 million years before the Cambrian era). There are fewer definitive fossils dating as far back as 800 million years, such as formations resembling the spicules of sea sponges.