Fossils unearthed in India that are 1.6 billion years old and look like red algae may represent the earliest-known plants, a discovery that could force scientists to reassess the timing of when major lineages in the tree of life first appeared on Earth.
Researchers on Tuesday described the tiny, multi-cellular fossils as two types of red algae, one thread-like and the other bulbous, that lived in a shallow marine environment alongside mats of bacteria. Until now, the oldest-known plants were 1.2-billion-year-old red algae fossils from the Canadian Arctic.
The researchers said cellular structures preserved in the fossils and their overall shape match red algae, a primitive kind of plant that today thrives in marine settings such as coral reefs but also can be found in freshwater environments. A type of red algae known as nori is a common sushi ingredient.
"We almost coul have had sushi 1.6 billion years ago," joked Swedish Museum of Natural History geobiologist Therese Sallstedt, who helped lead the study published in the journal PLOS Biology.
Earth formed about 4.5 billion years ago. There is evidence indicating life first appeared in the form of marine bacteria roughly 3.7 to 4.2 billion years ago. Only much later did plants and subsequently animals appear in the primordial seas.
"Plants have a key role for life on Earth, and we show here that they were considerably older than what we knew, which has a ripple effect on our appreciation of when advanced life forms appeared on the evolutionary scene," Sallstedt said.
The fossils were found in phosphate-rich sedimentary rocks from Chitrakoot in central India. The thread-like fossils contained internal cellular features including structures that appear to be part of the machinery of photosynthesis, the process used by plants to convert sunlight into chemical energy.
Oxygen is a byproduct of photosynthesis and the advent of plants helped build the atmosphere's oxygen content.
The fossils also contained structures at the center of each cell wall typical of red algae.
At the time, Earth's land surfaces were largely barren, life was mainly microbial and atmospheric oxygen was at 1 to 10 percent of current levels, said study co-leader Stefan Bengtson, a Swedish Museum of Natural History paleobiologist.
The fossils also represent the oldest-known advanced multi-cellular organisms in the broad category called eukaryotes that includes plants, fungi and animals, indicating complex life flourished much earlier than previously assumed, the researchers said.