The origin of animals as microbial host volumes in nutrient-limited seas

The microbe-stuffed gut, rather than the genome, represents the most dynamic gene reservoir within complex, multicellular metazoa (animals). Microbes are known to confer increased metabolic efficiency, increased nutrient recovery, and tolerance of ocean acidity to basal taxa such as sponges, arguably the extant taxa most comparable to the first metazoan. We hypothesize that metazoan origins may be rooted in the capability to compartmentalize, metabolize, and exchange genetic material with a modulated microbiome. We present evidence that the most parsimonious adaptive response of clonal eukaryotic colonies experiencing oligotrophic (nutrient-limited) conditions that accompanied Neoproterozoic glaciation events, which were broadly contemporaneous with metazoan origins, is to evolve a morphological volume to harbor a densified microbiome. Dense microbial communities housed within a cavity would increase instances of horizontal gene transfer between microorganisms and host, accelerating evolutionary innovation at the genetic and epigenetic levels for the holobiont. The accelerated tempo of genetic exchange would continue until the host’s metabolic and reproductive cells became spatially and temporally segregated from one another, at which point the process is effectively suppressed with the emergence of specialized gut and reproductive tissues. This framework may lead to new, testable hypotheses regarding metazoan evolution on Earth and a more tractable means of estimating the pervasiveness of complex, multicellular animal-like life with convergent morphologies on other planets.

The origin of animals as microbial host volumes in nutrient-limited seas

The microbe-stuffed gut, rather than the genome, represents the most dynamic gene reservoir within complex, multicellular metazoa (animals). Microbes are known to confer increased metabolic efficiency, increased nutrient recovery, and tolerance of ocean acidity to basal taxa such as sponges, arguably the extant taxa most comparable to the first metazoan. We hypothesize that metazoan origins may be rooted in the capability to compartmentalize, metabolize, and exchange genetic material with a modulated microbiome. We present evidence that the most parsimonious adaptive response of clonal eukaryotic colonies experiencing oligotrophic (nutrient-limited) conditions that accompanied Neoproterozoic glaciation events, which were broadly contemporaneous with metazoan origins, is to evolve a morphological volume to harbor a densified microbiome. Dense microbial communities housed within a cavity would increase instances of horizontal gene transfer between microorganisms and host, accelerating evolutionary innovation at the genetic and epigenetic levels for the holobiont. The accelerated tempo of genetic exchange would continue until the host’s metabolic and reproductive cells became spatially and temporally segregated from one another, at which point the process is effectively suppressed with the emergence of specialized gut and reproductive tissues. This framework may lead to new, testable hypotheses regarding metazoan evolution on Earth and a more tractable means of estimating the pervasiveness of complex, multicellular animal-like life with convergent morphologies on other planets.

Carbonaceous and siliceous Neoproterozoic vase-shaped microfossils (Urucum Formation, Brazil) and the question of early protistan biomineralization

Vase-shaped microfossils (VSMs) occur in dolomitic extraclasts of indeterminate provenance within the basal diamictite of the Neoproterozoic Urucum Formation (Jacadigo Group) of west-central Brazil, having an age constrained between 889±44 Ma (K-Ar; basement rocks) and 587±7 Ma (40Ar/39Ar age of early metamorphic cryptomelane in overlying manganese ore). Early isopachous carbonate cement entombed these VSMs, preserving rare direct evidence of original wall composition that is carbonaceous (now kerogenous) in practically all specimens. Some tests are siliceous or composed of a quartz-kerogen mixture; secondary replacement explains some features of these tests, but original biomineralization seems more likely for others. This interpretation, coupled with test morphology, suggests affinity to arcellinid testate amoebae. Five VSM taxa are recognized in the deposit:Cycliocyrillium simplexPorter, Meisterfeld, and Knoll, 2003, andC.torquataPorter, Meisterfeld, and Knoll, 2003, originally described in the Chuar Group (USA), and three new monospecific genera—Palaeoamphora urucumensen. gen. n. sp.,Limeta lageniformisn. gen. n. sp., andTaruma ratan. gen. n. sp. Most of the taxonomically important characteristics of these VSMs occur also in extant testate amoebae, but the combinations of some characters, such as organic-walled tests having exceptionally long necks that exhibit terminal apertures (L.lageniformisn. gen. n. sp.), are evidently novel additions to the known diversity of Neoproterozoic VSMs. Evidence of glacially influenced deposition in the conformably overlying Santa Cruz Formation may indicate that the Urucum Formation slightly preceded or was penecontemporaneous with a major Neoproterozoic glaciation, although the VSM-hosting extraclasts must be older, possibly rivaling the age of the testate amoebae of the Chichkan Formation (766±7 Ma) that are currently regarded as the oldest record of protists in the geological record.