Andrew Knoll, a professor of natural history at Harvard University, expressed similar reservations. “It is quite possible that the fossils reflect an extinct, early branching group,” he said in an email. “Various forms of multicellularity have evolved repeatedly within the greens, so such an interpretation isn’t a stretch.”
The evolutionary innovations seen in Tang’s fossil may have helped to set algae on a path that eventually led them ashore by about 470 million years ago. But the transition to land life would probably have begun hundreds of millions of years earlier, with green algae adapting to survive in damp soil or sand that was subject to temporary drying. Evolutionary biologists have generally believed that this transformation probably arose in parallel with the appearance of more complex multicellular structures, some of which lent themselves to these adaptations.
But evolution is not always a steady march forward. “Historically people have always assumed that evolution is a linear process—it keeps getting more and more complex. That’s been debunked time and time again, but it still permeates our thinking,” said Gane Ka-Shu Wong, a biologist at the University of Alberta.
In a recent paper in Cell, Wong, Michael Melkonian of the University of Duisberg-Essen in Germany, and their colleagues showed through genetic analysis that the closest living relatives to land plants are a mossy freshwater species known as Zygnematophyceae. Although these green algae are most often unicellular, they must have shared an ancestor with land plants: They have many genes in common that are crucial for survival on land, including some that confer resistance to drying out and some for synthesizing a cell wall. These commonalities suggest not only that many of the adaptations to dry land were gained before plants moved ashore, but also that some complex ancestral features were lost by the pond species over time when they ceased to be useful. “Evolution can be two steps forward, one step backward,” Wong said.
In a commentary on Wong and Melkonian’s paper in Current Biology, Philip Donoghue and Jordi Paps of the University of Bristol expand on the argument that many of the plant genes that look like adaptations to life on land seem to have much deeper histories and may have served different functions in algae originally. “Though the comparatively simple Zygnematophyceae may be the immediate relatives of the land plants, they are descended through simplification from more complex … ancestors whose body plans and genomes were assembled in deeper … ancestors,” they wrote.
In fact, some of those genetic histories may reach beyond algae: The Cell paper showed that the genes for surviving the stresses of desiccation may have originally come from soil bacteria and been donated to Zygnematophyceae (or their ancestors) through horizontal transfer.
In a separate study published in Current Biology last month, Paps, together with Alexander Bowles and Ulrike Bechtold of the University of Essex, compared the genomes of more than 200 living plants and used them to construct an evolutionary tree of their deep ancestry. In mapping the tree, they pinpointed when various genes emerged, and in the process they identified two bursts of extraordinary genomic novelty.
The second one, which seems to have occurred when algae were beginning to make the transition to terrestrial life, produced genes for adaptive features such as ultraviolet-light protection and the ability to form a root system and interact with terrestrial microbes. A much earlier burst occurred while the algae were still fully underwater and making the transition from unicellularity to multicellularity.
The discovery of that earlier burst shows some of the difficulties in unifying the billion-year-old fossils and the genetic results into a single, simple timeline of green plants’ evolution. To Tang and Xiao, the physical characteristics in the fossils suggest that species was a chlorophyte, one of the two main lineages of green algae—but not the one that most scientists think produced land plants. “It’s like we are talking about the granduncle of the land plants,” Xiao said. That ancient algae specimens therefore seem to have evolved their multicellular, complex structure earlier and independently of the burst of genetic creativity that Paps and his colleagues deduced from reconstruction.