G. Palyi, C. Zucchi, & L. Caglioti (eds) Fundamentals of Life 219-248. Elsevier, Paris.
Early diversification of organisms in the fossil record.

 Jerzy Dzik
Instytut Paleobiologii PAN, Twarda 51/55, 00-818 Warszawa, Poland. e-mail: dzik@twarda.pan.p

Abstract. Among the recent main contributions of palaeontology to the present understanding of the course of evolution are discoveries of fossil organisms transitional between basically different major groups of the living world. Thus, an almost complete transition from round worms to arthropods was documented with Cambrian fossils. This anticipated subsequently assembled molecular phylogenetic evidence that the ability of shedding out the chitinous cuticle is a very ancient trait defining the great superphylum Ecdysozoa. Another idea based on molecular evidence, that pelagic larvae are secondary adaptations in marine animals, the Lophotrochozoa, has found support in the Early Cambrian embryos with direct development within the egg covers. A surprising implication of the recent findings of chordates in the Early Cambrian is that these may represent the basalmost deuterostomes which possibly share some archaic aspects of their body with the nemerteans, Ediacarian dipleurozoans, and ctenophores. The latter were in the Cambrian more anatomically complex and diverse than today, and may have derived from the sedentary Ediacarian petalonameans. The main weakness of the available fossil evidence is its inability to document the common roots of animals and plants within the protists. Various Precambrian unicellular flagellates represented by cysts and cell scales, as well as multicellular algae, await an evolutionary interpretation. The cyanobacteria range in the fossil record prior to the formation of oxygen-rich atmosphere and molecular evidence suggests that they may be ancestral to all extant prokaryotes. This would mean that most of the bacterial evolution was a morphological simplification which followed their adaptation to heterotrophic and parasitic modes of life; also thermophily of the archaeobacteria being secondary.