Just as it fish took millions of year to come out of the water, green algae, the ancestors of terrestrial plants, clearly did not leave the sea from one day to the next. Researchers agree on this point. But exactly when did this decisive evolution take place? The question has already been much debated. And the article published in Science magazine (1), written principally by Philippe Steemans, palynologist at ULg, is likely to open the debate again. In his conclusions, the researcher asserts that the terrestrial ancestors of our trees and other vascular plants could be 30 million years older than we originally thought.

When algae came out of the water

The first forms of plant life on dry land are confirmed to have appeared at least 460 million years ago. These were very simple plants, close to certain types of algae, that were still highly dependent on an aquatic environment. They had no roots capable of taking water from the ground and their male sex cells had flagella since they still needed water to move around in search of female sex cells. They are called “bryophytes”. They left very discreet yet unquestionable traces of their presence on earth in the Ordovician period (which extended from 488 to 444 million years ago). These traces, called “cryptospores” – the cells that allowed these primitive plants to disperse – were easily fossilized. A microscope is required to study them because they barely measure several dozen microns, i.e. no bigger than the diameter of a hair. Nearly 500 million years later, in soil samples taken from the four corners of the earth, these fossilized cryptospores tell the story of the beginnings of life on dry land. But in order to find out what plants looked like during this period, scientists do not necessarily need to journey back in time because, believe it or not, bryophytes still exist today. If you have a small pond at the end of your garden, it may be surrounded by liverwort (a group of plants belonging to bryophytes in the broad sense), a small greenish plant barely measuring a few millimetres. These primitive plants form a carpet when they grow since they cannot grow upwards. But what a fantastic example of adaptation: they have been on earth for nearly 500 million years!

(1) Steemans, P., Le Herisse, A., Melvin, J., Miller, M.A., Paris, F., Verniers, J. and Wellman, C.H., 2009. Origin and radiation of the earliest vascular land plants. Science, 324: 353.