Precambrian Paleobiology: Precedents, Progress, and Prospects

Abstract

In 1859, C. R. Darwin highlighted the “inexplicable” absence of evidence of life prior to the beginning of the Cambrian. Given this lack of evidence and the natural rather than theological unfolding of life’s development Darwin espoused, over the following 50 years his newly minted theory was disputed. At the turn of the 19th century, beginning with the discoveries of C. D. Walcott, glimmerings of the previously “unknown and unknowable” early fossil record came to light – but Walcott’s Precambrian finds were also discounted. It was not until the breakthrough advances of the 1950’s and the identification of modern stromatolites (1956), Precambrian phytoplankton in shales (1950’s), stromatolitic microbes in cherts (1953), and terminal-Precambrian soft-bodied animal fossils (1950’s) that the field was placed on firm footing. Over the following half-century, the development and application of new analytical techniques coupled with the groundbreaking contributions of the Precambrian Paleobiology Research Group spurred the field to its international and distinctly interdisciplinary status. Significant progress has been made worldwide. Among these advances, the known fossil record has been extended sevenfold (from ∼0.5 to ∼3.5 Ga); the fossil record has been shown consistent with rRNA phylogenies (adding credence to both); and the timing and evolutionary significance of an increase of environmental oxygen (∼2.3 Ga), of eukaryotic organisms (∼2.0 Ga), and of evolution-speeding and biota-diversifying eukaryotic sexual reproduction (∼1.2 Ga) have been identified. Nevertheless, much remains to be learned. Such major unsolved problems include the absence of definitive evidence of the widely assumed life-generating “primordial soup”; the timing of the origin of oxygenic photosynthesis; the veracity of postulated changes in global photic-zone temperature from 3.5 Ga to the present; the bases of the advent of eukaryotic sexuality-requiring gametogenesis and syngamy; and the timing of origin and affinities of the small soft-bodied precursors of the Ediacaran Fauna.

Paleontological Evidence for Dinoflagellates and Ciliates as Early Eukaryotes

“…The fossil record of life begins in the early Archean (3500-3000 Ma) with an extremely long period of single-celled prokaryote microorganisms through to the Mesoproterozoic (1600-1000 Ma). The earliest microfossils are accepted to have been bacteria, followed by representatives of the cyanobacteria [36,37], based on comparative morphology and habitats with living organisms. Living bacteria and cyanobacteria are composed of biodegradable material not expected to fossilize, but many species of cyanobacteria are covered by a polysaccharide-rich sheath that sometimes may be preserved in Neoproterozoic rocks [38].…”

“…Living bacteria and cyanobacteria are composed of biodegradable material not expected to fossilize, but many species of cyanobacteria are covered by a polysaccharide-rich sheath that sometimes may be preserved in Neoproterozoic rocks [38]. The original direct evidence for their early occurrence comes from transparent thin sections of siliceous rocks (e.g., cherts) that formed under exceptional geochemical conditions [37]. Further evidence was added recently by discoveries of mineralized tubes in ancient fossil hydrothermal vents comparable to tubes produced by bacteria living in hydrothermal vents today [39].…”

“…The fossil record of early life changed abruptly around 2.1 billion years ago with the appearance of the earliest palynomorphs, the acritarchs [37,40]. This occurred within the period of early eukaryote evolution: one of the foremost advances in the history of life.…”

A movable beast: glaciation in the Ediacaran

“…(During Phanerozoic time, glaciation on Earth was limited to mid-to-high latitudes [ 4 ].) Large fossils resembling animals first appeared shortly after the last Snowball [ 5 ] but questions remain about the environment in which animal life first took hold. An enduring Ediacaran enigma is the occurrence of multiple glacial deposits on multiple continents.…”