8-1 Introduction

The understanding of the evolution of animals changed dramatically in the 19th century. In the early 19th century, William Paley published Natural Theology, which had four premises. 1) There is a divine creator. 2) Species did not ever change. 3) A short creation over 6,000 years. 4) There was a perfect design of each species.

Scientists began to uncover the fossil record of animals in the early 19th century. In journeys to South America and Indonesia, Darwin and Wallace detected small changes in species in slightly different environments, and ancient species that resembled modern species, and realized that change took place within and between species. Inspired by the limited resources concept of Malthus, Darwin and Wallace realized that natural selection of the strongest organisms would lead to changes in species. Darwin acknowledged that there were problems such as the suddenness of the Cambrian Explosion and sudden changes between species in the fossil record. Darwin thought that the earlier fossil record of the development of invertebrates would be discovered. He was correct.

Approximately 575 Ma, strange frondlike organisms began to appear in the seas in the Avalon Explosion. Sea anemones evolved within this group, which then evolved into acoelomates (flatworms) and pseudocoelomates (rotifers). Coelomates such as arthropods and mollusks began to rapidly evolve and diversify in the Cambrian Explosion 541 Ma, and nearly all coelomate phyla appeared in the fossil record by 518 Ma.

The growing consensus among paleontologists is that bilateria (vertebrates and invertebrates) evolved and diversified between 560 and 520 Ma (Section 8-3).[1] Most, if not all, of the 35 current animal phyla (groups) evolved during this period. Although complete fossils are rare, artifacts of animal activity from 560 to 520 Ma indicate the evolution of organs and the cavity (coelem) in which they are held, sensory organs, and various mechanisms of feeding and movement. Daley and Antcliffe made the following statement about the origin of the animal kingdom during this period.

"Stem lineages diversified during the Cambrian, leading to the construction of the modern crown groups, and originated no earlier than the very end of the preceding Ediacaran period, approximately 550 million years ago. As is the case with the arthropods, cnidarians and sponges, putative ctenophore fossils from the Ediacaran period were re-examined, and their ctenophore affinities rejected by Zhao and colleagues. Early molecular clock analyses suggested that there must be a deep Precambrian root for the origin of animals, but this no longer seems conceivable given the exhaustive approach taken to studying the fossil record, which has yet to recover any definitive Precambrian animal fossils. More recent molecular clock analyses also suggest a latest Ediacaran or early Cambrian origination of animals in line with these detailed paleontological studies. The ultimate goal in this field is to understand the mechanisms underlying the Cambrian Explosion, which requires both a clear timeline of the sequence of events and resolution of the interrelationships of the earliest diverging animal phyla." [2]

Section 8-4 describes early bilaterian animals: flatworms and rotifers. Section 8-5 describes the annelid phylum, which includes a broad range of segmented worms, including marine ragworms and earthworms. Section 8-6 describes the molluscs, which include the snails, clams, squids, and octopuses. Section 8-7 describes the arthropods, which include the trilobites and other crustaceans in the Cambrian Period. Approximately 120 million years after the beginning of the Cambrian, arthropod insects began to inhabit the land.

Section 8-8 describes the invertebrate deuterostomes, which include the starfish, sea urchins, amphioxus, acorn worms, and pterobranches. The first fossil evidence of the deuterostomes is in the early Cambrian Period; [3] however, the DNA molecular clock indicates a more ancient divergence of deuterostome vertebrates from invertebrates.

In Section 8-9 (optional excursus), Moses describes the natural origin of invertebrates in the sea as the first phase of the evolution of animals on earth.

References

[1] Budd, Graham E., and Sören Jensen. "The origin of the animals and a ‘Savannah’hypothesis for early bilaterian evolution." Biological Reviews 92, no. 1 (2017): 446-473.

[2] Daley, Allison C., Jonathan B. Antcliffe, Harriet B. Drage, and Stephen Pates. "Early fossil record of Euarthropoda and the Cambrian Explosion." Proceedings of the National Academy of Sciences 115, no. 21 (2018): 5323-5331.

[3] Giribet, Gonzalo, and Gregory D. Edgecombe. The Invertebrate Tree of Life. Princeton University Press, 2020.

Banner: Clam shell. Credit: febb. Used here per CC BY-SA 3.0.