Plants and animals may occupy distinct branches on the tree of life, but they could be more alike than we think.

Evolved Together?

In fact, green plants and animals enjoy a relatively close evolutionary relationship that has been obscured by a narrow focus on DNA sequences to find relatedness, says biologist John Stiller of East Carolina University.

Plants, fungi and animals are all in a group called the eukaryotes — distinguished by their advanced cellular machinery. But some eukaryotes, most notably the fungi, have long been considered more closely related to animals than plants are.

Stiller’s theory suggests organisms such as fungi should be given a demotion — placed further from animals on the tree — while green plants should get a leg up.

In a new paper on the subject, accepted for publication in the journal Trends in Plant Science, Stiller outlines the evidence. He told Discovery News that plants and animals have at least five features in common that could not have emerged independently.

"In both green plants and animals, cell cycles are controlled by master switches," he said. "These function, and malfunction, similarly in both groups."

As an example of a shared malfunction, Stiller pointed out that both groups suffer from cancerous growths consisting of rapidly diving cells that grow unchecked.

"The difference is that plants can often simply drop the growth in the way that they drop off their leaves, but humans and animals don’t possess that ability," he added.

Another attribute shared by plants and animals, according to Stiller, is the way the genetic material RNA operates in both groups. In both plants and animals, RNA acts as an intermediary between DNA and the protein it codes for. The enzymes that put RNA to work in a cell are similar in plants and animals, but not present in fungi or other organisms, he said.

And like animals, plants have an immune system. For the latter, Stiller argues that certain proteins and genes, which are not present in other organisms, help plants and animals defend themselves against invading viruses and bacteria.

Finally, and possibly most intriguingly, Stiller sees strong parallels between plant neurobiology and animal nervous systems.

"Plants obviously do not have actual nerves and brains, but electrical signals do allow plants to sense and to signal," he explained. "Some of the proteins involved in this process are the same in both groups."

"It is true we don’t have a firm idea of the closest relative of green plants," agreed Brent Mishler, a biologist at the University fo California at Berkeley and leader of The Green Tree of Life project, whose primary goal is to trace the lineage of green plants.

"But the green plant-animal hypothesis defended (by Stiller) is quite unusual and will take a compelling analysis to get people to believe it," he added.

Via: Discovery Channel