Plant Evolution: Overview
(Updated: February 17, 2004)
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Introduction to Plant Evolution The evolution of photosynthesis (about 3 billion years ago during the Precambrian) was, of course, a very important step towards the origin and evolution of ‘higher’ plants, and of other life forms on Earth. In particular, the origin of photosynthesizing bacteria, such as the cyanobacteria, and the subsequent origin of the eukaryotic lineage, the green algae, which had acquired chloroplasts through endosymbiosis, was critical for later stages of plant evolution. Polyploidy is also an important evolutionary process in the plant kingdom, due in part to the ability of plants to form viable (and sometimes fertile) hybrid progeny between closely related species. One of the major steps in plant evolution was the transition from an aquatic environment to a terrestrial environment. This was also an important step in animal evolution. It is believed that land plants evolved directly from a freshwater (versus a marine or salt-water) green algal ancestor sometime in the mid-Ordovician, between 450 to 470 million years ago (MYA) (Graham, 1993, p 234-235). The earliest known seedless vascular plant (genus: Cooksonia) existed in the mid-Silurian, probably around 420 MYA and eventually went exinct by the early Devonian (about 390 MYA) (ref. 3-4). Cooksonia was a small plant (only a few centimeters tall) classified under the Rhyniopsida, and had dichotomously branched leafless stems with terminal sporangia (ref. 4-6). Land plant megafossils older than Cooksonia have not been found, however, evidence from microfossils (e.g. pollen., spores) suggest that plants invaded the land around 450 to 470 MYA (Graham, 1993, p 8). This would have been an important step in the evolution of terrestrial animals -- the earliest land animals are reported to have existed about 414 MYA (Graham, 1993, p 13). Plants which colonised the land could have provided habitable environments (e.g. protection, and possibly food) for invading land animals. One of the difficulties faced by the first colonizers would be poor soil quality (or even the lack of soil). Land invasion would have had to have been gradual as the survival of later colonizers would depend on the nutrients provided by the earlier ones (i.e. early colonizers would die off and soil quality would gradually improve as time went on). A look at the differences between aquatic and terrestrial environments might be useful in understanding land invasions. This comparison applies not just to land plants, but also to other terrestrial organisms such as animals, fungi, and certain algae. The following table summarizes the major features of modern aquatic and terrestrial environments.
There are a number of consequences for living in either type of environment, including those of osmoregulation, structural support, and reproductive strategies. Many features that were adapted for aquatic environments may have been useful in the transition towards inhabiting terrestrial environments (these are reviewed in Graham, 1993).
The major land plant groups arose and diverged between 400 to 300 MYA, during the Devonian and Carboniferous peroids. The angiosperms probably arose sometime in the late Jurassic to the early Cretaceous (roughly 140 MYA), and then they underwent a peroid of major diversification about 90 MYA (mid-Cretaceous), eventually becoming the dominant group of land plants that we see today (consisting of over 250 000 species across 350 families; Kenrick, 1999). The origin and evolution of angiosperms has been under much debate, however, as noted by Hughes (1994, p 21), “angiosperm origins comprise a problem, not a mystery”. The same may also be said of other areas in plant evolution (or in biology and in evolution in general) -- problems can and will be solved by research (it just takes time and money to do so). That is, after all, what research is about.
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