A Brief History of Life on Earth

The first multicellular life develops many years after single celled organisms. It is unclear exactly how or why this happens, but one possibility is that single-celled organisms go through a stage similar to that of modern choanoflagellates: single-celled creatures that sometimes form colonies consisting of many individuals. Of all the single-celled organisms known to exist, choanoflagellates are the most closely related to multicellular animals, lending support to this theory.

Evolution of multicellularity 900 mya

Angiosperms produce a flower containing male and/or female reproductive structures. Early in seed-plant evolution, insects became pollen carriers as they searched for food. In turn, plants evolved floral nectar and odors for attracting insects to carry pollen. One hypothesis of angiosperm derivation cites the extinct cycadeoids (seed ferns) as its ancestor. Fossil evidence indicates that flowering plants first appeared in the Lower Cretaceous. Some indirect evidence suggests origin from 200 mya but Angiosperm fossils of that age are unknown, probably because they evolved in dry, uplands that were not conducive to fossilization.

Origin of Angiosperms 125 mya

Metazoans (animals) are eukaryotes that acquire energy by ingesting other organisms (heterotrophy). Most animals, representing the vast majority of metazoan phyla, are multicellular, have bilateral symmetry, and possess specialized cells organized into tissues and organs. Animals originated from the division of eukaryotes into different lineages. DNA evidence comparing all three eukaryote lineages has been most supportive of this theory.

Origin of animals 1.5 bya

The evolution of fish began during the Cambrian explosion, as supported by fossil evidence. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish.The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today), although perhaps the reverse is the case.

Origin of fish 530 mya

A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. Fungi evolved from unicellular protists and to become multicellular organisms. DNA evidence suggests that almost all fungi have a single common ancestor. They were probably aquatic organisms with a flagellum.

Origin of fungi 1.5 bya

The gymnosperms are a group of seed-producing plants that includes conifers, cycads, Ginkgo, and gnetophytes. It is widely accepted that the gymnosperms originated in the late Carboniferous period, replacing the lycopsid rainforests of the tropical region.This appears to have been the result of a whole genome duplication event around 319 million years ago. The fossil plant Elkinsia polymorpha, a "seed fern" from the Devonian period (about 400 million years ago) is considered the earliest seed plant known to date.

Origin of gynmosperms 400 mya

Insects evolved from a group of crustaceans. The first insects were land bound, but about 400 million years ago in the Devonian period one lineage of insects evolved flight, the first animals to do so. The oldest definitive insect fossil, Rhyniognatha hirsti, is estimated to be 407 to 396 million years old.

Origin of insects 400 mya

Plants evolved from green algae and invaded the vacant land niche. Many adaptations were needed as plants moved from water onto land, as well as many symbiotic relationships. Evidence on the origin and diversification of land plants has come mainly from dispersed spores and megafossils.

Origin of plants 1.5 bya

The Cambrian Period marks an important point in the history of life on Earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion," because of the relatively short time over which this diversity of forms appears. Before complex animals could evolve on Earth, there had to be an environment favourable for their survival. Researchers have examined a number of environmental factors that might have been instrumental in the evolution of new body plans, but the two strongest contenders are a rise in oxygen levels and the end of extreme glacial conditions.

The Cambrian Explosion 535 mya