Scientists may be one step closer to determining when exactly photosynthesizing plants (Viridiplantae) first evolved, with the discovery of a new species of ancient algae they have named Proterocladus antiquus.
The fossilized remains of the plant, described in Nature Ecology & Evolution, were discovered on the 1,000 million-year-old Nanfen Formation in the northern province of Liaoning, China, which shares a border with North Korea.
At around a billion years old, it is believed to be one of the oldest examples of a multicellular chlorophyte—a group of green algae from the division Chlorophyta, which are thought to have played a key role in pre-Mesozoic (225 million years ago and older) ecosystems. Despite its old age, it is consistent with a lot of green algae that is still found today—for example, it contains multiple cells and root-like structures.
"If our interpretation is correct, then P. antiquus from the ca.one billion-year-old Nanfen Formation represents one of the earliest known multicellular chlorophytes," they write.
Green plants able to extract energy from the sun's rays (photosynthesis) are thought to have emerged sometime between the Palaeoproterozoic era (2,500–1,600 million years ago) and the Cryogenian period (720–635 million years ago). Without their evolution, the development of more complex life—including humans— would have been impossible. However, a dearth of evidence on the fossil record has made it hard to identify their evolutionary timeline with much more accuracy.
The hope is that the dating of the newly discovered P. antiquus will enable scientists to narrow down the window by creating a minimum age for the split between Rhodophyta (red algae) and Viridiplantae and the evolution of multicellularity in Chlorophyta, among other things.
The study's authors say P. antiquus is differentiated from other species of Proterocladus by certain features, including its root-like structure and upward-growing lateral branches. However, they caution that the fragmentary nature of the remains could mean a previously identified species of Proterocladus (such as P. minor, found in Svalbard, Norway) has been mistaken for an entirely new species.
To put the discovery into perspective, most research pins the evolution of multicellular organisms to a period approximately 600 million years ago, the Ediacaran Period—though remains of what is thought to be red algae has been dated to 1.6 billion years ago. The first vertebrates started to emerge some 130 million years later, during the Ordovician Period, approximately 470 million years ago, when algae is thought to have been the only multicellular plant on the planet.
Land plants did not start to appear until the Silurian Period, 430 million years ago, after a mass extinction took out a large proportion of marine life. Flowers did not evolve until much later—120 million years ago.
