The conquest of the land by plants was one of the most important developments in the history of life on Earth, paving the way for animals. Yet scientists have been quite uncertain about when this hugely significant event took place. A new study claims to have the answer, providing a much earlier figure than most previous estimates.
The oldest confirmed fossilized land plants that we are confident of date to 427 million years old. However, since the first colonizers were not giant hardwoods, it is reasonable to think the early versions did not fossilize well. We also have some hard-to-date fossil plants that could be much older. A team at the University of Bristol in the UK argue in the Proceedings of the National Academy of Sciences that; “The sparseness of early land plant megafossils and stratigraphic controls on their distribution make the fossil record an unreliable guide,” to when plants first arrived on land.
The alternative is to use “molecular clocks”. These use differences in DNA to show how long it has been since different living species separated. It is thought that a single common ancestor colonized the land, and the species we see today broke away from each other relatively soon thereafter. However, molecular clocks require an understanding of the family tree on which the work is done, and that is something we lack.
Botanical geneticists have disagreed on the relationships between the four primary categories of land plants, and which separated from the others first. Seven alternative structures are considered serious possibilities as representatives of how these four relate. Rather than getting caught up in worrying which theory is correct, Dr Jennifer Morris and colleagues constructed timescales based on each of these, using the fossil records we are confident of as calibrators. They found that whichever model is used, the earliest separation among land plants occurred between 515 and 469 million years ago, 40-90 million years before fossil-based estimates.
The spread of land plants should have greatly changed the planetary climate, both by directly drawing carbon dioxide out of the air, and by increasing the weathering of rocks. “Previous attempts to model these changes in the atmosphere have accepted the plant fossil record at face value – our research shows that these fossil ages underestimate the origins of land plants, and so these models need to be revised,” said Morris in a statement.
Co-lead author Mark Puttick added; “Our results show the ancestor of land plants was alive in the middle Cambrian Period (509-497 million years ago).” The findings explain the mystery that tracks that were apparently made by early land animals are around 450 million years old, yet it is hard to explain how they could have preceded plants onto the land.
