Tiny plant-like fossils reveal that advanced multicellular life may have existed for much longer than previously thought.
The earliest traces of life on Earth date back at least 3.5 billion years and consist of single-celled organisms. Until now, it was thought that large multicellular organisms with more complex “eukaryotic” cells 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, presenting challenges for scientists trying to reconstruct and date the tree of life.
Therese Sallstedt, a geobiologist at the Swedish Museum of Natural History, was part of the team of researchers that discovered two kinds of fossils resembling red algae in uniquely well-preserved sedimentary rocks in Chitrakoot, central India. Characteristic thread-like forms were discovered first, and when Sallstedt investigated further, she found more complex, fleshy algae structures. The presumed red algae lie embedded in fossil mats of cyanobacteria, called stromatolites, in 1.6 billion-year-old Indian phosphorite — making them the oldest plant-like fossils ever found.
To tell us more about this study, Sallstedt agreed to answer a few questions via email.
Tell me a little bit about your background. What led you to study paleobiology?
I have always been interested in questions related to the origin of life on Earth and possibly elsewhere in the universe. For me personally, it was reading a popular science article about life in extreme environments and their biosignatures when I was a biology student. [This] prompted me to try and pursue a science career path. I changed my major to geology with the intent to try and find a Ph.D. project after graduation. It was simply far too interesting not to. Now, I want to figure out how we best can interpret the signatures left for us in the rock record by organisms that lived billions of years ago.
Organisms this ancient can be hard to identify because there is no DNA remaining. How did you determine that the fossils you found were red algae?
We used a variety of microscopy methods, of which synchrotron based microtomography was particularly valuable, to examine the fossils in three dimensions. This allowed us to study both cellular and subcellular structures within the specimens. Together with the overall appearance, size and cellular organization, the subcellular features we identified suggested the fossils were red algae.
What does your study tell us about the tree of life?
Our study suggests that complex multicellular eukaryotes — plants in particular — are a lot older than what was previously assumed. It suggests a deeper divergence of advanced life on Earth, meaning we may have to revise the time scale of the so-called tree of life to accommodate these ancient eukaryote fossils.
What is next for you and your research?
We have plenty of samples left from India and plan to continue our research on these kinds of early plant fossils. Also, I hope to further my own research on ancient sedimentary environments to also accommodate the deep biosphere and ocean crust as a fossil record into deep times.
Research Article: Bengtson S, Sallstedt T, Belivanova V, Whitehouse M (2017) Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae. PLoS Biol 15(3): e2000735. doi:10.1371/journal.pbio.2000735
Image Credit: Stefan Bengtson