Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian

Self Cite

Three-dimensionally preserved embryos from the Precambrian Ediacaran Doushantuo Formation, Weng'an, Guizhou, southern China, have attracted great attention as the oldest fossil evidence yet found for multicellular animal life on Earth. Many embryos are early cleavage embryos and most of them yield a limited phylogenetic signal. Here we report the discovery of two Doushantuo embryos that are three-dimensionally preserved and complex. Imaging techniques using propagation phase-contrast based synchrotron radiation microtomography (PPC-SR-CT) reveal that the organization of cells demonstrates several bilaterian features, including the formation of anterior-posterior, dorso-ventral, and right-left polarities, and cell differentiation. Unexpectedly, our observations show a noticeable difference in organization patterns between the embryos, suggesting that they represent two distinct taxa. These embryos provide further evidence for the presence of bilaterian animals in the Doushantuo biota. Furthermore, these bilaterians had already diverged into distantly related groups at least 40 million years before the Cambrian radiation, indicating that the last common ancestor of the bilaterians lived much earlier than is usually thought.early animal evolution ͉ early metazoan evolution ͉ nondestructive 3-D reconstruction ͉ precambrian ediacaran embryos ͉ Weng'an biota

Section: Discussionsupporting

confidence: 80%

Complex embryos displaying bilaterian characters from Precambrian Doushantuo phosphate deposits, Weng'an, Guizhou, China

Chen

Bottjer

et al. 2009

Self Cite

“…However, the interpretation of these fossils as bilaterian metazoans has been questioned, and they have been reinterpreted as encysted holozoan protists (6). Similar problems plague the earliest reported adult bilaterian, Vernanimalcula, which is also from the Doushantuo Formation of southern China (7,8). Fossils of Vernanimalcula (∼200 μm in diameter) have been described as broadly oval and triploblastic with a mouth, a differentiated gut surrounded by paired coeloms, and an anus.…”

Section: Fossil Recordmentioning

confidence: 99%

Evolution of centralized nervous systems: Two schools of evolutionary thought

2012

108

Understanding the evolution of centralized nervous systems requires an understanding of metazoan phylogenetic interrelationships, their fossil record, the variation in their cephalic neural characters, and the development of these characters. Each of these topics involves comparative approaches, and both cladistic and phenetic methodologies have been applied. Our understanding of metazoan phylogeny has increased greatly with the cladistic analysis of molecular data, and relaxed molecular clocks generally date the origin of bilaterians at 600-700 Mya (during the Ediacaran). Although the taxonomic affinities of the Ediacaran biota remain uncertain, a conservative interpretation suggests that a number of these taxa form clades that are closely related, if not stem clades of bilaterian crown clades. Analysis of brain-body complexity among extant bilaterians indicates that diffuse nerve nets and possibly, ganglionated cephalic neural systems existed in Ediacaran organisms. An outgroup analysis of cephalic neural characters among extant metazoans also indicates that the last common bilaterian ancestor possessed a diffuse nerve plexus and that brains evolved independently at least four times. In contrast, the hypothesis of a tripartite brain, based primarily on phenetic analysis of developmental genetic data, indicates that the brain arose in the last common bilaterian ancestor. Hopefully, this debate will be resolved by cladistic analysis of the genomes of additional taxa and an increased understanding of character identity genetic networks.The fact that some of these building stones are universal does not, of course, mean that the organs to which they contribute are as old as these molecules or their precursors.von Salvini-Plawen and Mayr (1) A ny consideration of the evolution of centralized nervous systems is inextricably linked to an understanding of the phylogeny of living metazoans, their fossil history, the vast range of complexity in their nervous systems, and the development of these nervous systems. For this reason, any attempt to reconstruct the phylogeny of metazoan CNSs must be based on all lines of evidence available. The molecular phylogenetic studies of the last 20 y are particularly important in understanding metazoan interrelationships as well as the time frame in which these animals arose and radiated, and we now have increased insights into the genetics underlying the development of CNSs.First, I will review the fossil history of the earliest putative metazoans, and then, I will discuss different comparative approaches to analyzing both molecular and morphological data: the molecular clock hypothesis, which has yielded a range of possible dates for the origin and divergence of metazoans; developmental genetics and its contribution to our understanding of the patterning of metazoan bodies, particularly patterning of the CNS; and conclusions based on the first outgroup analysis of metazoan central neural characters. Finally, I will review two hypotheses concerning the morphological complexity of the ...

“…Although there have been many glacial events recorded in the history of the Earth (9-11), two major periods of low-latitude glaciation in the Proterozoic appear correlated with significant changes in the evolution of life (8,(12)(13)(14) and atmospheric oxygen level (15)(16)(17)(18)(19). The Paleoproterozoic Makganyene glaciation occurred approximately between 2.3 and 2.2 Ga, and at least two other low-latitude glaciations occurred during the Cryogenian period, between Ϸ740 and 630 Ma (9,10,20).…”

mentioning

confidence: 99%

Production of hydrogen peroxide in the atmosphere of a Snowball Earth and the origin of oxygenic photosynthesis

Liang

Hartman

Kopp

et al. 2006

103

During Proterozoic time, Earth experienced two intervals with one or more episodes of low-latitude glaciation, which are probable ''Snowball Earth'' events. Although the severity of the historical glaciations is debated, theoretical ''hard Snowball'' conditions are associated with the nearly complete shutdown of the hydrological cycle. We show here that, during such long and severe glacial intervals, a weak hydrological cycle coupled with photochemical reactions involving water vapor would give rise to the sustained production of hydrogen peroxide. The photochemical production of hydrogen peroxide has been proposed previously as the primary mechanism for oxidizing the surface of Mars. During a Snowball, hydrogen peroxide could be stored in the ice; it would then be released directly into the ocean and the atmosphere upon melting and could mediate global oxidation events in the aftermath of the Snowball, such as that recorded in the Fe and Mn oxides of the Kalahari Manganese Field, deposited after the Paleoproterozoic low-latitude Makganyene glaciation. Low levels of peroxides and molecular oxygen generated during Archean and earliest Proterozoic non-Snowball glacial intervals could have driven the evolution of oxygen-mediating and -using enzymes and thereby paved the way for the eventual appearance of oxygenic photosynthesis.atmospheric processes ͉ Paleoproterozoic ͉ photochemistry