An investigation of 890-million-year-old limestone rocks excavated from the mountains of Canada's Northwest Territories has revealed possible evidence of the oldest animal life on Earth. Prior to this study, the oldest undisputed fossils of animal life have been those of ancient sponges, or poriferans (members of the phylum Porifera), that lived approximately 540 million years ago during the Cambrian period. However, scientists analyzing thin sections of the Northwest Territories rocks identified three-dimensional tubelike structures that are similar to the structural network of certain modern sponges, thereby provisionally pushing back the beginnings of the earliest sponges by about 350 million years. If the findings of this study are confirmed, they would indicate the presence of animal life on Earth prior to the Neoproterozoic Oxidation Event, which was a period occurring between 800 million and 540 million years ago that boosted atmospheric oxygen levels closer to modern-day levels. Thus, the putative ancient sponges were likely tolerant of comparatively low oxygen levels, suggesting that the origins of animal life on Earth may have been less dependent on oxygen. See also: Animal evolution; Evolution; Fossil; Limestone; Oxygen; Paleontology; Petrography; Petrology; Porifera; Precambrian
Hundreds of millions of years ago, much of the land comprising today's North America was covered by warm, shallow seas. The aforementioned 890-million-year-old fossils were found during an excavation of an ancient microbial reef created by marine cyanobacteria (collectively, a bacterial group of photosynthetic microorganisms, formerly mislabeled as blue-green algae). In contrast to most present-day reefs, which are derived from corals, microbial reefs are built in layered fashion by cyanobacteria. When microbial reefs become fossilized, they are referred to as stromatolites. Stromatolites are the oldest macroscopic evidence of life on Earth, at least 3.5 billion years old. Importantly, stromatolite laminations are useful indicators of various forms of life present in ancient sedimentary rocks, especially forms of life that are often difficult to preserve in the fossil record. See also: Cyanobacteria; Microbial ecology; North America; Reef; Sedimentary rock; Stromatolite
The fossilized tubelike structures observed in the 890-million-year-old rocks resemble the meshlike networks of modern-day keratosan sponges (members of the subclass Keratosa in the poriferan class Demospongiae). In contrast to most modern-day sponges, which have skeletons composed of mineralized spicules, keratosan sponges lack these rigid structures. Instead, keratosans have branching networks of flexible tubes made from fibrins of spongin—a protein with a soft, spongelike texture. Because fibrous materials are not as readily preserved in the taphonomic fossil record as rigid mineralized structures, keratosans appear less frequently in the fossil record and thus fail to provide researchers with conclusive evidence to support theories of ancient life. The investigators responsible for analyzing the Northwest Territories rocks believe that the fossilized meshlike networks are uniquely indicative of sponges. Other paleontologists not involved in the study are more skeptical of the findings, believing that ancient bacteria, algae, fungi, plant life, or even other unidentified animals may be responsible for creating the fossilized branching networks. Therefore, further comparisons to other fossilized sponges as well as modern-day sponges are required to support the tentative conclusions regarding these possibly oldest remnants of animal life. See also: Demospongiae; Protein; Taphonomy
