4th Transition Convention

The evolution (about 1.5 billion years ago) of complex nucleated eukaryotic cells, like those found in plants and animals, coincided with increased levels of oxygen in the atmosphere. The fact that these cells contained nuclei was only one aspect of their complexity; they also possessed internal subcellular structures - organelles - for performing specific tasks. The single-celled protozoa and algae, and all multicellular life, evolved from these cells.

The earliest traces of multicellular animals are rare imprints of soft-bodied invertebrate animals resembling jellyfish, segmented worms, and sea pens in rock from around 600 million years ago, toward the end of the Precambrian period. In general, only hard parts of an organism - shells, scales, spicules and (later) bones - become fossilized. So the fossil record is incomplete and highly selective, incorporating only very rare traces of the many entirely soft-bodied animals and plants that must have existed. Invertebrate animals with hard parts started to appear at the beginning of the Cambrian, the period that saw an explosion of animal life in the oceans. By the end of the Cambrian period, all of the main varieties of animal types - whose descendents fill the world today - had been established. Corals, sea urchins, bivalve molluscs and many-limbed trilobites flourished and proliferated. Fish - the first vertebrate animals - began to dominate the seas. Yet still the seas were the only theaters of life; there was still no life on land.

Plants appeared on land some 400 million years ago in the Devonian period. They evolved into a range of types, from small mosses to giant tree ferns. Land animals - insects, air-breathing lungfish and the first amphibians - began to populate the land. Reptiles and winged insects appeared in the Carboniferous period. The latter evolved in concert with flowering plants (whose fertilization the insects made possible). Successive periods of prehistoric life led to the gradual colonization by plants and animals of most of the niches available on Earth. Significant prehistoric events include the appearance of freshwater fish, the rise of reptiles, the age of dinosaurs, the flight of the first birds, and the appearance of mammals.

Small mammals had lived with dinosaurs for millions of years. They first emerged at approximately the same time as dinosaurs (about 200 million years ago), but there were few vacant niches for them, since land habitats were permeated with dinosaurs. A global catastrophe changed the course of evolution 65 million years ago. Most likely, a massive object from space collided with Earth, resulting in a major climate disturbance. The mammals were less affected than the more specialized dinosaurs, and became ascendant. In time, the mammals diversified to include the variety that we are familiar with today, including hoofed mammals and the primates.