The origins and radiations of metazoans, bilat- erians, deuterostomes and vertebrates have been recognized as the four most important milestone events in animal his- tory and, conversely four of the toughest and puzzling ques- tions in evolutionary biology. Among them, the first two oc- curred in the deep time of the Precambrian and have left little information in the fossil records. Fortunately, intensive investigation into the Early Cambrian Chengjiang faunas in the past two decades have revealed numerous significant data on the main episode of the Cambrian Explosion, par- ticularly information on the various earliest-known deu- terostomes, leading to a much better understanding of the last two events. The “first fish” Haikouichthys and Myllo- kunmingia have been proved the oldest-known vertebrates, and to be among the most primitive ones, representing a key transitional group from acraniates to craniates. Indeed, “Pa- leontological work of Shu and collaborators is revising our understanding of the early evolution of chordates”. Moreover, the discovery of the Phylum Vetulicolia may also throw new light on the origin of deuterostomes. This extinct group with simple gill slits might represent one of the roots in the deu- terostome lineage, as the evidence of molecular biology and developmental biology has predicted. Here the first discovery of tiny vetulicolians from the Chengjiang Lagerst?tte is re- ported. The evolutionary trend from the giant to the small might represent a special adaptation in early animal history. Since all vetulicolians share similar characters in their ante- rior section (pharynx), and on the basis of differences in their posterior section, a new classification system is proposed, with a new class, the Heteromorphida erected. As both the “root” group vetulicolians and the “top” group vertebrates in the early deuterostome lineage, as well as those members in between, such as vetulocystids, yunnanozoans, cephalo- chordates and urochordates, have been recovered, a rela- tively complete early deuterostome phylogeny, which is compatible with phylogenies of extant deuterostomes, is nearly mature. The origins and radiations of metazoans, bilat- erians, deuterostomes and vertebrates have been recognized as the four most important milestone events in animal his- tory and, conversely four of the toughest and puzzling ques- tions in evolutionary biology. Among them, the first two oc-curred in the deep time of the Precambrian and have little information in the fossil records. Fortunately, intensive investigation into the Early Cambrian Chengjiang faunas in the past two decades have shown numerous significant data on the main episode of the Cambrian Explosion, par- ticularly information on the various earliest-known deu- terostomes, leading to a much better understanding of the last two events. The “first fish” Haikouichthys and Myllo- kunmingia have been proved the oldest-known vertebrates, and to be among the the most primitive ones, representing a key transitional group from acraniates to craniates. Indeed, “Pa leontological work of Shu and collaborators is revising our understanding of the early evolution of chordates ”. Moreover, the discovery of the Phylum Vetulicolia may also throw new light on the origin of deuterostomes. This extinct group with simple gill slits might represent one of the roots in the de- terostome lineage, as the Here the first discovery of tiny vetulicolians from the Chengjiang Lagerst? tte is re- ported. The evolutionary trend from the giant to the small might represent a special adaptation in early animal history. Since all vetulicolians share similar characters in their ante ri ri section (pharynx), and on the basis of differences in their posterior section, a new classification system is proposed, with a new class, the Heteromorphida erected. As both the “root” group vetulicolians and the “Top” group vertebrates in the early deuterostome lineage, as well as those members in between, such as vetulocystids, yunnanozoans, cephalo- chordates and u rochordates, have been recovered, a rela- tively complete early deuterostome phylogeny, which is compatible with phylogenies of extant deuterostomes, is nearly mature.