Earth’s oldest eukaryotic fossils may illuminate both the rise of complex life and the search for life elsewhere.
Searching for life on Mars or the icy moons Europa and Enceladus may attract more attention than studying how life developed on Earth. Yet tracing the appearance of the first eukaryotes on our planet is just as important to astrobiology, especially because microbial organisms dominated roughly 90 percent of Earth’s history.
Life originated on Earth more than 3.5 billion years ago, Ross Anderson, a paleontologist at the University of Oxford in the U.K., explained. Cyanobacteria and oxygen-producing photosynthesis were present at least 2.3 billion years ago, followed by eukaryotes no later than 1.7 billion years ago, he said.
Algae appeared at least one billion years ago and probably earlier. Animals emerged at least 570 million years ago, and possibly somewhat before then.
To reach the shared ancestor of plants and animals, however, researchers must look back approximately 1.6 billion years, according to Anderson.
So-called crown eukaryotes, which represent the earliest members of major modern eukaryotic groups, played a central role in the rise of complex life on Earth. Anderson considers eukaryotes to be the planet’s first truly complex organisms.
Eukaryotes made complex life possible
Eukaryotes have a cell nucleus where DNA is enclosed, but they also have organelles, subcellular structures in their cells, such as the mitochondrion, that allows energy-intensive lifestyles.
It’s the eukaryotes which have developed complex multicellularity and macroscopic forms; all the animals, plants and fungi that we see around the world are eukaryotic, says Anderson.
Soft bodies leave a sparse record
No organism older than 500 million years had shells and skeletons, as they hadn’t evolved yet. As a result, paleontologists are reliant on quite unusual environmental settings where cellular remains and soft tissues can be preserved. Consequently, researchers know very little about how life was evolving across a period which makes up 90 percent of Earth’s history.
“I’m interested in how we went from a planet which just had bacteria to one which had complex multicellular organisms,” says Anderson. “Those kinds of multicellular fossils are hard to find, so I do a lot of work on the chemistry of the rocks to find out in which settings they are preserved.”
One problem is that eukaryotic microfossils are subject to billions of years of degradation.
“But we know that the transition from single-celled to multicellular happened multiple times on different parts of the Earth, says Anderson. We’re interested in how animals became so diverse today,” he says.
Most of their diversity got set up across the Ediacaran/Cambrian transition. That’s an epoch some 540 million years ago that represented a major evolutionary step between soft-bodied biota and the Cambrian explosion of life with mobility, shells, and skeletons.
Remote rocks preserve rare clues
As for where to look for such ancient eukaryotic microfossils?
Anderson and colleagues are particularly interested in a 100 sq km area in what was a shallow sea, some 80 degrees North in a remote group of islands near Svalbard, Norway.
And just last year, in Australia, researchers found some of the oldest eukaryote microfossils ever discovered, dating back some 1.75 billion years.
The sweet spot for such microfossils usually is in ancient coastal areas, where eukaryotes would have had access to rich organics and plentiful nutrients, enabling them to grow in both multicellularity and diversity.
The idea is to go to areas that are either pristine or have not been sampled to any great extent. Anderson himself specializes in looking in areas that were subject to massive clay deposits that might have helped preserve these ancient eukaryotes.
Today, you’re looking at places that are desert or Arctic, where there’s no vegetation so rocks are exposed, says Anderson.
None of which is easy. Looking for eukaryote microfossils is truly a herculean task, because they are both tiny and consist of unprotected soft tissue, which has left them subject to massive degradation over billions of years.
Earth guides the search elsewhere
“The biggest challenge now is that we have an undersampled fossil record,” says Anderson.
“We’ve started to figure out which are the right rocks to find early fossils, and that’s starting to give us the data with which we can record the history of Earth’s earliest life,” says Anderson.
“A lot of the work we’ve done on clays was motivated by finding life on other planets,” says Anderson. “We’d better understand how life happened here if we hope to understand the likelihood of it happening elsewhere.”
Adapted from an article originally published in UniverseToday.
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