Survival and selection biases in early animal evolution and a source of systematic overestimation in molecular clocks

Abstract

Important evolutionary events such as the Cambrian Explosion have inspired many attempts at explanation: why do they happen when they do? What shapes them, and why do they eventually come to an end? However, much less attention has been paid to the idea of a ‘null hypothesis’—that certain features of such diversifications arise simply through their statistical structure. Such statistical features also appear to influence our perception of the timing of these events. Here, we show in particular that study of unusually large clades leads to systematic overestimates of clade ages from some types of molecular clocks, and that the size of this effect may be enough to account for the puzzling mismatches seen between these molecular clocks and the fossil record. Our analysis of the fossil record of the late Ediacaran to Cambrian suggests that it is likely to be recording a true evolutionary radiation of the bilaterians at this time, and that explanations involving various sorts of cryptic origins for the bilaterians do not seem to be necessary.

Diversification dynamics of total-, stem-, and crown-groups are compatible with molecular clock estimates of divergence times

“…Here, the age of the crown-group was fixed to the age of the oldest undisputed crown-arthropod [514 Ma; (10,18,19)], a conservative estimate of the age of crown-arthropods. Initially, the speciation rate was fixed such that the expected extant diversity of 7,000,000 species (11) achieved over 535 million years-a span of time chosen to reflect a conservative paleontological estimate of duration of the arthropod total-group (20). We show that total-group ages are exponentially distributed with the upper 95th percentile at 641.1 Ma; this result is robust to changes in the clade age used to estimate the speciation rate.…”

Taphonomic experiments imply a possible link between the evolution of multicellularity and the fossilization potential of soft‐bodied organisms

“…Speculating on the role of CAMs and ECMs in preservation, we need to ask an important question. According to molecular phylogenies, the first multicellular animals must have appeared long before the Cambrian Explosion, probably as early as 740–800 Ma (Budd & Mann, 2020; Dohrmann & Wörheide, 2017). Why there is no rich fossil record of these early animals?…”

The origin and rise of complex life: progress requires interdisciplinary integration and hypothesis testing

“…If chromium isotopes do indicate there was a rise in atmospheric O 2 levels from limiting low levels [2], then it happened several times in the Mesoproterozoic and Neoproterozoic [34], and it is unclear when it became persistent [34]. Inevitably, there is debate over the timing of the origin of animals linked to the veracity of both molecular clock estimates and the fossil record on which they rely for calibration [7,35,36]. Integrating over this uncertainty is challenging since the breadth of estimates from this range of approaches and perspectives spans the late Tonian-middle Ediacaran.…”

“…If so, for a given atmospheric pO 2 , the highest [O 2 ] would be expected in relatively shallow, permanently (tidally) well-mixed shelf seas, at cooler higher latitudes [32]. Once again, uncertainties in the interpretation of the fossil record and the evolutionary timescales that we can ultimately derive from it [7,35,36] currently preclude effective tests [7].…”

“…Budd & Mann [36] build on these early records of animals and their absence from older strata to challenge molecular timescales for Cryogenian or Tonian estimates of animal antiquity. They reason that the fossil record exhibits a stratigraphically consistent phylogenetic structure, with the earliest possible animal fossils first occurring in the middle Ediacaran, followed by the earliest possible eumetazoans in close succession, with trace and body fossil evidence indicating an origin of crownbilaterians at or around the Proterozoic-Phanerozoic boundary.…”