Origin and early evolution of the vertebrates: New insights from advances in molecular biology, anatomy, and palaeontology

Abstract

Recent advances in molecular biology and microanatomy have supported homologies of body parts between vertebrates and extant invertebrate chordates, thus providing insights into the body plan of the proximate ancestor of the vertebrates. For example, this ancestor probably had a relatively complex brain and a precursor of definitive neural crest. Additional insights into early vertebrate evolution have come from recent discoveries of Lower Cambrian soft body fossils of Haikouichthys and Myllokunmingia (almost certainly vertebrates, possibly related to modern lampreys) and Yunnanozoon and Haikouella (evidently stem-group vertebrates). The earliest vertebrates had an unequivocally marine origin, probably evolved mineralised pharyngeal denticles before the dermal skeleton, and evidently utilised elastic recoil of the visceral arch skeleton for suction feeding. Moreover, the new data emphasise that the advent of definitive neural crest was supremely important for the evolutionary origin of the vertebrates.

Early crest animals and the insight they provide into the evolutionary origin of craniates

“…The so-called ''gut contents, '' however, lay outside, but not within, the ''gut.'' The ''gut'' and ''gut contents'' interpretations are not grounded by evidence but likely are misidentifications of the notochord and protovertebrae Li, 1997, 2000;Holland and Chen, 2001;. In the same article, Shu et al had proposed the presence of ''gill pores'' and a ''stomochord'' in Yunnanozoon, which are, however, misinterpretations of the branchial heart and endostyle, respectively.…”

“…The branchial arches measure about 80 lm in diameter in larval sea lamprey and only 30 lm in amphioxus. They are numerously jointed and consist of numerously stacked discs that resemble those of the branchial arches made by cartilage cells in larval lamprey and embryonic fishes (Holland and Chen, 2001;Kimmel et al, 1998;Pardos and Benito, 1988;. It suggests that the branchial arches are cellular, composed of cartilage.…”

“…They represent water chambers on the sides of the pharynx for receiving exhalent ventilation water leaving the branchial slits of the pharyngeal lumen and for conveying the water to the outside through the atriopore in cephalochordates or an exhalent siphon in tunicates. The atria might function for protecting the delicate branchial arches from muddy and sandy substratum during swimming and burrowing (Holland and Chen, 2001) and for protecting the delicate gill rays from damage by water currents during swimming.…”

“…The presence of jointed cartilage branchial arches, pharyngeal teeth (Figs. 1C,, and the evidence of muscular ventilation suggest that Haikouella and Yunnanozoon had definitive neural crest cells and were definitive crest animals (Holland and Chen, 2001;. Cristozoa thus include craniates and their immediate precursors, e.g., precraniate Cristozoa.…”

Chordate origins and evolution

“…In this volume, Chen discusses the insights provided by the controversial Haikouella and Yunnanozoon fossils collected from the Doushantuo formation in China. These Precambrian fossils were dated to about 530 Mya (millions of years ago) and have been alternatively interpreted as a chordates, hemichordates, basal deuterostomes or precraniate representatives of Cristozoa (Chen, 2008 and references therein), a new proposed subphylum that includes all animals that display neural crest cells or their derivatives (Holland and Chen, 2001). The peculiar state of preservation of his material allows Chen to confront alternative hypotheses and to propose that both fossils provide missing links to an ancestral Chordate, which he pictures, in agreement with the Olfactores hypothesis, as an animal similar to the cephalochordate amphioxus.…”