EVOLUTIONARY ORIGINS
OF LIFE CYCLES

Reproduction in prokaryotes, such as bacteria, is very different than in most eukaryotes. Prokaryotes cannot reproduce sexually, and therefore have limited opportunity for genetic recombination -- the mixing of genetic information that creates variety in organisms. The exchange of genetic information in prokaryotes is limited to three basic processes: (1) transformation allows genes to be taken up from the surrounding environment; (2) conjugation is a direct transfer of genes from one bacterium to another via cellular connection; and (3) transduction is the insertion of genes by a virus. All of these processes have undoubtedly played a major role in the evolution of prokaryotes.

Plants have come a long way since the Precambrian Era and have developed complex structures to assure the survival of future generations. Their life cycles are now composed of processes that give them advantages over plants from the past. To go from prokaryotes to angiosperms is to take a giant leap in plant diversity, not only in morphology, but also as far as cycles of life are concerned.

When Eukaryotes appeared, a new door opened up for the development of life cycles. Although they are often unicelled and simple, some algae have a 1n-2n life cycle which constitutes an alternation of generations. Reproductively, this method is greatly advantageous to an organism genetically and ecologically.

Land plants all share this 1n-2n reproduction, but changes in structure allowed certain plants to attain greater levels. The separation of vascular plants from bryophytes is a key point in evolutionary history. Bryophytes reproduce through syngamy and meiosis, but the dominant stage of the plant is the gametophyte, as opposed to the sporophyte in vascular plants.

The earliest of the vascular plants are the pteridophytes and lycopods. These include the ferns and the club mosses. The lack of seeds is a key difference between these organisms and the gymnosperms and angiosperms. In addition, the sperm of these plants flagellate, but it rarely happens in the seed plants.

A link between the pteridophytes and conifers has been found in the progymnosperms. These organisms reproduced like pteridophytes, but looked like gymnosperms. The adaptation of seeds let the gymnosperms reach new heights in plant life cycle.

Gymnosperms reproductively broke away from nonseed plants in a few key ways. First, they adopted cones to help protect the spores in meiosis and fertilization. While plants were mostly homosporous before, gymnosperms became completely heterosporous and took in the advantages of seeds. Changes included the reduction of the gametophyte and the absence of antheridia.

The angiosperms took development in the life cycle one step further. The seeds became enclosed, whereas the seeds in gymnosperms are naked. Cones were modified to inflorescences, flowers and fruits. Lastly, archegonia are always absent in angiosperms. These changes allowed them to diversify even further into monocotyledons and dicotyledons. When looking at an apple tree, it is amazing to think that an organism so complicated derived from a relatively simple one.