Evolution of Micro-organisms and Plants

Silurian Period, 443.4 - 419.2 MYA

During this period plants continued to adjust to the environmental niche on land. Those that grew on riverbanks had a convenient supply of water but were out of the reach of foraging fish. Minerals were available in moist soils. Then the land became an extraordinary environmental niche for animals, because the soil-rooted plants themselves offered food for the taking. There was no competition for the first animals capable of making forays onto the land; the land was unoccupied. Thus abundant animal fossils are found only in strata laid down after plants became common on land. Actually, it took about 50 million years before animals developed ways to incorporate plants as food sources. Plants are low in protein, high in carbohydrate; a large proportion of the latter is in the form of cellulose and animals do not possess the enzyme to break it

down. Such obstacle of coping with cellulose and the shortage of protein has been overcome in most herbivorous insects and vertebrates by the evolution of a symbiotic relationship with micro-organisms in the gut where they digest the plant material, providing a nutritious brew for their host. Figure 14 shows that at the end of the Silurian period, swamps and marshes beside the sea were already occupied by low vegetation composed of the most primitive types of vascular cryptogams (Psilophytales), plants that reproduce, like fern, by wind-dispersed spores. Some genera were leafless, but others already bore leaves. On the right, in the water, is the Zosterophyllum (a form between ferns and club mosses); to the left of that, on the land, grows the tall Psilophyton, with the low, creeping, spidery Sciadophyton below it. On the left, by the water, is Proto-lepidodendron, the forerunner of the present club mosses, and in the background are clumps of the creeping Drepanophycus, which looked rather like a club moss.