Tag Archives: meristem

Roots evolved at least twice — independently

A new study found that modern roots evolved at least twice, slowly taking the familiar shapes of today.

Convergent evolution

Despite popular belief, evolution doesn’t cherry pick the best traits to develop. It’s more of a random development taking place, and if the change is good, then the individual has more chances to pass its genes on — and it starts to become the norm. Sometimes, a trait is so useful that different species in different times or places develop it independently. This is called convergent evolution.

A classic example of convergent evolution is flight. Birds and bats are completely unrelated, yet they’ve both developed flight, a trait that defines both groups (even though they do it differently). Now, researchers have found another example of convergent evolution: roots.

It’s hard to imagine a world without plants, but they weren’t always around (at least, as we know them today). Land plants evolved from a group of green algae, perhaps as early as 850 years ago, but roots developed much later. The earliest root impressions date from the Late Silurian, some 420 million years ago (after sharks had developed as a group, for context). The evolution of roots came with dramatic global consequences. They disturbed the soil and absorbed nutrients such as nitrate and phosphate, promoting soil acidification. This, in turn, enabled the weathering of deeper rocks, injecting carbon compounds deeper into the soils — which changed the Earth’s climate. It’s believed that this change might have even led to a mass extinction.

But for plants, there’s no denying it: roots were an amazing thing.

Old Roots

Sandy Hetherington and Liam Dolan, the authors of a new study, studied the so-called Rhynie chert — a 407 million-year-old sedimentary deposit that contains exceptionally well-preserved remains of the oldest known terrestrial ecosystem. Located in Aberdeenshire, Scotland, this chert formation holds various remarkable plant fossils including lichen and roots.

Examining chert samples under the microscope, the two have found pieces of a tissue called meristem, belonging to a plant called Asteroxylon mackiei — an extinct vascular plant, related to today’s club mosses.

The meristem is the defining feature of modern-day plant roots — a self-renewing structure comprising of undifferentiated cells capable of growing into a variety of different tissues, depending on the plant’s needs.

But here’s the thing: the meristem in Asteroxylon mackiei appears to be a transitional stage towards the more modern type of meristem. But since a relative of Asteroxylon mackiei had already developed modern meristem, it seems that the feature had developed independently before.

The study has been published in Nature.