BALTIMORE — Microscopic vampires may have prowled ancient seas some 750 million years ago. Scientists have found the fossilized remains of their punctured victims. Those fossils may be the oldest direct evidence of predators hunting eukaryotes (Yu-KAIR-ee-oats). This domain of complex life includes both plants and animals.
The monstrous microbes probably didn’t look like tiny Count Draculas. But, “they’re just as terrifying, at least if you’re a single-celled organism,” said Susannah Porter. She is a paleontologist at the University of California, Santa Barbara. She spoke here November 1 at the Geological Society of America's annual meeting.
The predators poked holes in their prey. Afterward, they slurped the victim’s juicy innards, Porter proposes. This predation helped drive single- and multicelled eukaryotes to evolve innovations, she suggests. The new developments might have included skeletons and the ability to burrow.
The early emergence of predation makes sense, says Paul Falkowski. He is an Earth systems scientist at Rutgers University in New Brunswick, N.J. “Once you make an organism, somebody else will figure out a way to utilize its juices.”
What the fossils showed
Eukaryotes first appear in the fossil record around 1.8 billion years ago. They started rapidly diversifying, some billion years later. The emergence of predators likely helped to spark this burst of evolution. Identifying predation in the fossil record is tricky, however.
Porter examined 750-million-year old fossils from the Grand Canyon. She used a scanning electron microscope. As she pored over the cells, she noticed something odd. Several fossils had clean-cut circular holes in their cell walls. Some specimens contained 30 or more punctures.
Hole-riddled fossils are nothing new. But the wounds in these fossils seemed deliberate, Porter said. The holes ranged in size from 0.2 to 2.9 micrometers in diameter. That’s really small — around the thickness of a red blood cell. The holes in any one fossil, though, were always roughly the same size. Some holes were even beveled. Their gaps narrowed toward the fossil’s interior.
This evidence pointed to predation, Porter said. Other processes, such as mineral growth, would have made different types of holes. The disappointing news: No fossils of the predators themselves have been found.
Several modern microbe species exhibit similar hole-punching behavior. Among them are some predatory amoebas. They belong to an aptly named genus: Vampyrella. Scientists think these microbes use enzymes to eat away a hole through a victim’s cell wall. The amoeba then sucks out the cell’s contents. It may even slip through the opening to eat the cell from the inside out.
With vampire-like predators haunting the seas 750 million years ago, any one eukaryote species would have had trouble holding dominance, Porter said. When predators are around, organisms find it in their interest to develop new ways to avoid getting noshed. That helps to maintain a high diversity of organisms.
A predation-sparked arms race probably led some eukaryotes to don defensive scale-like armor. Fossils showing that innovation emerged around this time. “It’s stupid to make armor unless you’re defending against a predator,” Falkowski says. “Organisms that have no predation pressure will just have very simple membranes facing the outside world.”
Porter agrees: “Predation is such a strong selective pressure: You don’t want to die.”