A maple tree is clearly very different from a moss, yet over a long period of time, and step by step, early moss-like plants evolved to trees, including maples. The fossil record tells us about some of these steps, and experts have agreed upon the probable steps where the fossils don’t tell us. The story centers on solving problems involved with the invasion of land from the sea or fresh water.<\/p>\n
Hundreds of millions of years ago, green algae began to colonize land. They already had the mechanisms of photosynthesis: using light to combine carbon dioxide and water into nutritious carbohydrates. And they already had sexual reproduction, which produces new combinations of genes every generation and thus the variation necessary for evolutionary change.<\/p>\n
Experts say, and there are hints in the fossil record, that the very early land plants formed associations with fungi that provided nutrients, a mutualistic association that almost all land plants have maintained. So those early terrestrial plants were off to a good start.<\/p>\n
[Don’t be afraid to fail, especially at shrimping]<\/a><\/ins><\/p>\n For sexual reproduction, however, those early plants were entirely dependent on water. Eggs and sperm were released into water, where sperm could swim a few centimeters to find a drifting egg to fertilize. That’s OK for plants living in water or even in damp soil, but it won’t work on dry land.<\/p>\n The first land plants are thought to have been more or less moss-like — small plants growing close to the ground. If conditions were not wet enough, these early plants had to wait for sex until conditions improved. Although most modern mosses still have to wait, as did the early plants, a few are reported to use springtails, mites or flies to transfer sperm to eggs, but few such animals were present in the early days of the invasion of land.<\/p>\n But even if sperm could swim to an egg, the resulting zygote would be still on its own in a potentially dangerous environment. Somewhere along the line, although sperm was released and still had to swim, eggs began to be retained in special structures on the leaves of the parent plant. Fertilization then occurred within these special structures and the zygote received both protection and some nutrition during development.<\/p>\n [Sights, sounds and smells of spring]<\/a><\/ins><\/p>\n In mosses, that zygote stays on its mother and grows into a new individual (called a sporophyte) that looks different from its mother and eventually produces spores. Spores are single cells inside a tough coat that disperse on the wind, and, if they land in a good spot, grow into new mosses (called gametophytes, because they produce gametes). Thus, the generations alternate between gamete production and spore production.<\/p>\n There was still the problem of needing water for the sperm to swim to an egg. The fossil record is poor at this point, but clearly, at some point, moss-like plants began to produce two kinds of spores on their sporophytes: small, male spores with sperm, and big, female spores with eggs.<\/p>\n The big spores did not disperse but were retained on their mother sporophyte’s leaves, where they received protection and nutrition. The small spores ultimately developed more protective covers; they traveled on the wind and found special landing places near the female spores, where they could fertilize the eggs.<\/p>\n In some cases, that special landing place is a droplet of fluid, close to the egg, that engulfs the arriving male spore (if it belongs to the same species as the egg) and pulls it in. That is how pollination came about. The big, female spores began to stock nutritional material inside, for the growth of the embryo, and that was the beginning of the evolution of seeds.<\/p>\n