Scientists have discovered 1.6 billion-year-old fossils of red algae in central India, which may be the oldest evidence of plant-like life found on the Earth. The finding in Chitrakoot, Madhya Pradesh by researchers from the Swedish Museum of Natural History indicate that advanced multicellular life evolved much earlier than previously thought. The earliest traces of life on Earth are at least 3.5 billion years old. These single-celled organisms, unlike eukaryotes, lack nuclei and other organelles. Large multicellular eukaryotic organisms became common much later, about 600 million years ago, near the transition to the Phanerozoic Era, the "time of visible life." Discoveries of early multicellular eukaryotes have been sporadic and difficult to interpret, challenging scientists trying to reconstruct and date the tree of life. The oldest known red algae before the present discovery are 1.2 billion years old.
The Indian fossils, 400 million years older and by far the oldest plant-like fossils ever found, suggest that the early branches of the tree of life need to be recalibrated. The scientists found two kinds of fossils resembling red algae in uniquely well-preserved sedimentary rocks at Chitrakoot. One type is thread-like, the other one consists of fleshy colonies. The scientists were able to see distinct inner cell structures and so-called cell fountains, the bundles of packed and splaying filaments that form the body of the fleshy forms and are characteristic of red algae. "You cannot be a hundred per cent sure about material this ancient, as there is no DNA remaining, but the characters agree quite well with the morphology and structure of red algae," said Stefan Bengtson, Professor at the Swedish Museum of Natural History. "The 'time of visible life' seems to have begun much earlier than we thought," said Bengtson. The presumed red algae lie embedded in fossil mats of cyanobacteria, called stromatolites, in 1.6 billion-year-old Indian phosphorite. The thread-like forms were discovered first, and when researchers investigated the stromatolites they found the more complex, fleshy structures. The research group was able to look inside the algae with the help of synchrotron-based X-ray tomographic microscopy. Among other things, they have seen regularly recurring platelets in each cell, which they believe are parts of chloroplasts, the organelles within plant cells where photosynthesis takes place. They have also seen distinct and regular structures at the centre of each cell wall, typical of red algae. The research was published in the journal PLOS Biology.
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