MadSci Network: Evolution |
EXCELLENT question!!! Another question would be: Was meiosis and sex "invented" just once? Or did plants, animals, fungi and other organisms each start out as haploids and invent sex independently? I am not an expert on this topic, but as far as I know, meiosis and sex was invented just one time, and all organisms that use meiosis today are descended from that one ancestor. This would place the date back some 2 billion years, before fungi, plants, protozoans and animals split from a common ancestor. There are other types of "sex", or more correctly, other ways for similar organisms to exchange genes. For example, bacteria have plasmids and phages that can carry genes from one bacterial lineage to another, and influenza viruses have segmented genomes which allow some recombination. An interesting fact that many people do not know about, is that many sexual organisms which use haploid and diploid genomic states just as mammals do, do not have just 2 sexes, nor do they spend the majority of their life cycle in the diploid state. Fungi, for example spend most of their time haploid, and only briefly enter the diploid state. Some fungi have hundreds of different sexes rather than just 2. Most plants are diploid for the majority of the time, but many of the plants are both male and female. Even within the animals which have male and female sexes, their is a lot of variety in how the sex is determined. Most animals do not use the XX = female and XY = male chromosome system that primates use. Aligators determine sex by temperature at which their eggs incubate, for example, so both males and females have the same chromosomes. Honeybees use different food given to the embryo to make drones, workers and queens. It seems quite likely that sex was one of the critical "inventions" that allowed the more rapid creation of the most diverse life forms on earth today. Although bacteria and other asexual mono-cellular life forms have more diversity in basic metabolism and biochemistry than the plants and animals and fungi, the multi-cellular organisms with sex have the greatest diversity of morphologies and the most rapid changes in morphology over time. But even within a single group of organisms, there can be huge differences in those rates of change. For example beetles and ants are both insects, but the beetles have taken on much more diverse morphologies than the ants. All ants look pretty much alike except for differences in size and color, whereas beetles have all sorts of shapes and lifestyles as well as differences in size and color. Even if meiosis and sex was only "invented" once, it has been "forgotten" many times. Many organisms have lost the ability to reproduce sexually and now reproduce only by cloning themselves. It happens quite often in the plant kingdom for example. If I am wrong about sex only being invented once, please write to me and let me know. Brian Foley btf@t10.lanl.gov
Try the links in the MadSci Library for more information on Evolution.