Chemical fossils tag Precambrian sponges
A new NSF-funded study has isolated chemical biomarkers of marine sponges dating to 635 million years ago, during Neoproterozoic times. Dating from about 100 million years prior to the Cambrian Explosion of animal diversity, this new chemical data demonstrates that at least one line of sponge animals, a group called the demosponges, were already around back then. The new evidence doesn’t overturn anything, because independent genetic studies have long predicted that the roots of animal diversity probably extend at least that deep in time. However, this new study provides clear evidence that simple true animals were actually there…. it firmly tags them on the geologic time scale.
The new study by Gordon Love of the University of California at Riverside, and colleagues, found traces of a group of compounds called steranes in 635 million year old marine sedimentary rocks from Oman. Steranes are a category of organic compounds associated with steroids manufactured by eukaryotic creatures – those that have cell nuclei and mitochondria. The particular steranes found by Love and coworkers are uniquely identified with demosponges today.
The new data show that despite failing to preserve large physical imprints of intact animals – the type of fossil most people think about when they hear the word fossil – the rocks examined in this study retained chemical traces of ancient life. The molecules produced by living animals and plants are just as revealing as physical imprints… in some cases more revealing, because fossil imprints record form but rarely substance. Chemical biomarkers allow geologists to identify particular cell lines in ancient rocks. With this kind of data one can stick a pin into the map of evolution through time and mark it with the name of a specific gene lineage. Chemical biomarkers are a powerful technique in studying the history of life.
The timing of these new fossils places them at roughly the end of an episode known as Snowball Earth, when most of the Earth’s surface was covered in ice, including probably the equator. Most paleontologists consider the Snowball Earth event to have been a major trigger of the abrupt proliferation of animal diversity recorded in rocks from immediately after those times. The frozen Earth would have killed off a lot of microbial life, which was the dominant form of life at that point. A worldwide die-off of microbial life and primitive proto-animals would have opened lots of habitat range for post-Snowball pioneering survivors, possibly leading to an explosive radiation of new species and animal forms that culminated in the Cambrian Explosion.