The assassin's bullet misses, the Archduke's carriage moves forward, and a catastrophic war is avoided. So too with the history of life. Re-run the tape of life, as Stephen J. Gould claimed, and the ...outcome must be entirely different: an alien world, without humans and maybe not even intelligence. The history of life is littered with accidents: any twist or turn may lead to a completely different world. Now this view is being challenged. Simon Conway Morris explores the evidence demonstrating life's almost eerie ability to navigate to a single solution, repeatedly. Eyes, brains, tools, even culture: all are very much on the cards. So if these are all evolutionary inevitabilities, where are our counterparts across the galaxy? The tape of life can only run on a suitable planet, and it seems that such Earth-like planets may be much rarer than hoped. Inevitable humans, yes, but in a lonely Universe.
Agnathan fish hold a key position in vertebrate evolution, especially regarding the origin of the head and neural-crest-derived tissue. In contrast to amphioxus, lampreys and other vertebrates ...possess a complex brain and placodes that contribute to well-developed eyes, as well as auditory and olfactory systems. These sensory sytems were arguably a trigger to subsequent vertebrate diversifications. However, although they are known from skeletal impressions in younger Palaeozoic agnathans, information about the earliest records of these systems has been largely wanting. Here we report numerous specimens of the Lower Cambrian vertebrate Haikouichthys ercaicunensis, until now only known from the holotype. Haikouichthys shows significant differences from other fossil agnathans: key features include a small lobate extension to the head, with eyes and possible nasal sacs, as well as what may be otic capsules. A notochord with separate vertebral elements is also identifiable. Phylogenetic analysis indicates that this fish lies within the stem-group craniates. Although Haikouichthys somewhat resembles the ammocoete larva of modern lampreys, this is because of shared general craniate characters; adult lampreys and hagfishes (the cyclostomes if monophyletic) are probably derived in many respects.
The first fossil chordates are found in deposits from the Cambrian period (545-490 million years ago), but their earliest record is exceptionally sporadic and is often controversial. Accordingly, it ...has been difficult to construct a coherent phylogenetic synthesis for the basal chordates. Until now, the available soft-bodied remains have consisted almost entirely of cephalochordate-like animals from Burgess Shale-type faunas.
While the broad framework of deuterostome evolution is now clear, the remarkable diversity of extant forms within this group has rendered the nature of the ancestral types problematic: what, for ...example, does the common ancestor of a sea urchin and lamprey actually look like? The answer to such questions can be addressed on the basis of remarkably well-preserved fossils from Cambrian Lagerstätten, not least the celebrated Chengjiang Lagerstätte (Yunnan, China). This deposit is particularly important because of its rich diversity of deuterostomes. These include some of the earliest known representatives, among which are the first vertebrates, as well as more enigmatic groups, notably the vetulicolians and yunnanozoans. The latter groups, in particular, have been the subject of some radical divergences in opinion as to their exact phylogenetic placements. Here, we both review the known diversity of Chengjiang deuterostomes and in particular argue that the vetulicolians and yunnanozoans represent very primitive deuterostomes. Moreover, in the latter case we present new data to indicate that the yunnanozoans are unlikely to be any sort of chordate.
Ediacaran assemblages immediately predate the Cambrian explosion of metazoans and should have played a crucial role in this radiation. Their wider relationships, however, have remained refractory and ...difficult to integrate with early metazoan phylogeny. Here, we describe a frondlike fossil, Stromatoveris (S. psygmoglena sp. nov.), from the Lower Cambrian Chengjiang Lagerstätte (Yunnan, China) that is strikingly similar to Ediacaran vendobionts. The exquisite preservation reveals closely spaced branches, probably ciliated, that appear to represent precursors of the diagnostic comb rows of ctenophores. Therefore, this finding has important implications for the early evolution of this phylum and related diploblasts, some of which independently evolved a frondose habit.
Cambrian fossil-Lagerstätten (sites of exceptional fossil preservation), such as those from Chengjiang (Lower Cambrian) and the Burgess Shale (Middle Cambrian), provide our best window into the ...Cambrian 'explosion'. Such faunas are known from about 40 localities, and have yielded a widely disparate series of taxa ranging from ctenophores to agnathan fish. Recent excavations of the Chengjiang fossil-Lagerstätte, known from a series of sites near Kunming in Yunnan, south China, have resulted in the discovery of several new forms. In conjunction with material described earlier, these provide evidence for a new group of metazoans, the vetulicolians. Several features, notably a series of gill slits, suggest that this group can throw light on an early stage of deuterostome diversification.
Articulated halkieriids of Halkieria evangelista sp. nov. are described from the Sirius Passet fauna in the Lower Cambrian Buen Formation of Peary Land, North Greenland. Three zones of sclerites are ...recognizable: obliquely inclined rows of dorsal palmates, quincuncially inserted lateral cultrates and imbricated bundles of ventro-lateral siculates. In addition there is a prominent shell at both ends, each with radial ornamentation. Both sclerites and shells were probably calcareous, but increase in body size led to insertion of additional sclerites but marginal accretion of the shells. The ventral sole was soft and, in life, presumably muscular. Recognizable features of internal anatomy include a gut trace and possible musculature, inferred from imprints on the interior of the anterior shell. Halkieriids are closely related to the Middle Cambrian Wixaxia, best known from the Burgess Shale: this clade appears to have played an important role in early protostome evolution. From an animal fairly closely related to Wixaxia arose the polychaete annelids; the bundles of siculate sclerites prefigure the neurochaetae whereas the dorsal notochaetae derive from the palmates. Wixaxia appears to have a relic shell and a similar structure in the sternaspid polychaetes may be an evolutionary remnant. The primitive state in extant polychaetes is best expressed in groups such as chrysopetalids, aphroditaceans and amphinomids. The homology between polychaete chaetae and the mantle setae of brachiopods is one line of evidence to suggest that the latter phylum arose from a juvenile halkieriid in which the posterior shell was first in juxtaposition to the anterior and rotated beneath it to provide the bivalved condition of an ancestral brachiopod. H. evangelista sp. nov. has shells which resemble those of a brachiopod; in particular the posterior one. From predecessors of the halkieriids known as siphogonuchitids it is possible that both chitons (polyplacophorans) and conchiferan molluscs arose. The hypothesis of halkieriids and their relatives having a key role in annelid—brachiopod—mollusc evolution is in accord with some earlier proposals and recent evidence from molecular biology. It casts doubt, however, on a number of favoured concepts including the primitive annelid being oligochaetoid and a burrower, the brachiopods being deuterostomes and the coelom being an archaic feature of metazoans. Rather, the annelid coelom arose as a functional consequence of the transition from a creeping halkieriid to a polychaete with stepping parapodial locomotion.
Deuterostomes are a remarkably diverse super-phylum, including not only the chordates (to which we belong) but groups as disparate as the echinoderms and the hemichordates. The phylogeny of ...deuterostomes is now achieving some degree of stability, especially on account of new molecular data, but this leaves as conjectural the appearance of extinct intermediate forms that would throw light on the sequence of evolutionary events leading to the extant groups. Such data can be supplied from the fossil record, notably those deposits with exceptional soft-part preservation. Excavations near Kunming in southwestern China have revealed a variety of remarkable early deuterostomes, including the vetulicolians and yunnanozoans. Here we describe a new group, the vetulocystids. They appear to have similarities not only to the vetulicolians but also to the homalozoans, a bizarre group of primitive echinoderms whose phylogenetic position has been highly controversial.
Clearly, the fossil record from the Cambrian period is an invaluable tool for deciphering animal evolution. Less clear, however, is how to integrate the paleontological information with molecular ...phylogeny and developmental biology data. Equally challenging is answering why the Cambrian period provided such a rich interval for the redeployment of genes that led to more complex bodyplans.
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