Microfossil prey containing puncture holes were discovered in the Grand Canyon.
If dinosaur bones seem hard to find, then try searching for fossils that are too tiny to be seen with the naked eye.
This is the usual challenge for microfossil hunter Susannah Porter from the University of California at Santa Barbara, who recently discovered rocks from the eastern Grand Canyon containing evidence of the earliest predation on eukaryotes — organisms whose cells contain a membrane-bound nucleus — that occurred between 780-740 million years ago.
Because the fossils are so small, Porter didn’t realize she had found any until she returned home and had the samples processed. “It can be a bit of a gamble doing Precambrian paleontology, but I lucked out!” she told The Science Explorer.
What she saw when she peered at the remains of the ancient single-celled organisms under the microscope was a series of circular holes drilled into their cell walls. The holes were similar in shape and size to those made by modern predatory protists known as ‘vampire amoebae’, which find their way into eukaryotic cells and feed on their contents.
These tiny vampires certainly don’t have fangs. Though it is not known exactly how they puncture cell walls, according to Porter, it seems likely that they produce enzymes that break the walls down.
Heavy predation over time might have placed pressure on eukaryotes to develop defenses. “We are interested in seeing if perhaps the walls of the prey get thicker through time, with increased predation,” said Porter, noting that the recent discovery of carbon compounds preserved in the Precambrian rocks suggests that ancient eukaryotes were using chemical defenses.
But the evolutionary consequences of early predation on eukaryotes may have been far greater.
Porter describes a previous study in which a predatory microorganism was introduced into cultures of single-celled green algae. Within 1,000 generations, the green algae population evolved multicellularity. The introduced predator consumed prey by swallowing it whole, so once the green algae became multicellular, they were too large to be swallowed.
“This suggests that multicellularity — and the larger size that goes along with it — might be have evolved as defense against predation,” said Porter.
Alternatively, it has been suggested that multicellularity evolved in the ancestor to animals as a means of more effectively capturing bacterial prey. “So multicellularity could have evolved both as a defense and a weapon,” she said.
Though this research, published in Proceedings of the Royal Society B, describes the oldest direct evidence of predation on eukaryotes, older examples may still abound in other rock assemblages. Porter plans to explore these older rocks to see just how far back the record goes.
She would also like to understand how the early environment might have enabled these predators to take off. Recent work suggests that the Cambrian explosion — a period in which most major animal groups appear for the first time in the fossil record — was driven by predators that were only able to thrive once oxygen levels got high enough to support them.
“I'm curious to know whether something similar occurred with their single celled ancestors ~200 million years before,” said Porter.
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