All sexually reproducing eukaryotic organisms (animals, plants and fungus) evolved from a common ancestor that was a single celled eukaryotic species. Many protists reproduce sexually, as do multicellular plants, animals, and fungi. There are a few species which have secondarily lost this feature, such as Bdelloidea and some parthenocarpic plants. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. However, since the hypotheses for the origins of sex are difficult to test experimentally, most current work has been focused on the maintenance of sexual reproduction.
It seems that a sexual cycle is maintained because it improves the quality of progeny (fitness), despite making the creation of offspring less likely (the two-fold cost of sex). In order for sex to be evolutionarily advantageous, it must be associated with a significant increase in the fitness of offspring. One of the most widely accepted explanations for the advantage of sex lies in the creation of genetic variation. Another explanation is based on two molecular advantages. The first of these is the advantage of recombinational DNA repair (promoted during meiosis because homologous chromosomes pair at that time), while the second is the advantage of complementation (also known as heterosis, hybrid vigor or masking of mutations). Read more ...
Rethinking Early Evolution: Earth's Earliest Animal Ecosystem Was Complex And Included Sexual Reproduction Science Daily - March 20, 2008
Two paleontologists studying ancient fossils they excavated in the South Australian outback argue that Earth's ecosystem has been complex for hundreds of millions of years -- at least since around 565 million years ago, which is included in a period in Earth's history called the Neoproterozoic era. Until now, the dominant paradigm in the field of paleobiology has been that the earliest multicellular animals were simple, and that strategies organisms use today to survive, reproduce and grow in numbers have arisen over time due to several factors. These factors include evolutionary and ecological pressures that both predators and competition for food and other resources have imposed on the ecosystem. But in describing the ecology and reproductive strategies of Funisia dorothea, a tubular organism preserved as a fossil, the researchers found that the organism had multiple means of growing and propagating -- similar to strategies used by most invertebrate organisms for propagation today. on."
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