The corpses of the victims are roughly 750 million years old.
An autopsy of their fossilized remains attests to gruesome deaths. The single-celled organisms, discovered in the cliffs of the Grand Canyon, are covered in tiny holes—telltale signs of microscopic vampiric attacks by predators that punctured their outer skin and then ate their innards, piece by piece.
“They’re clever little organisms,” says Susannah Porter, a paleobiologist at the University of California, Santa Barbara, who recently published a postmortem study of the microbial feeding frenzy in the journal Proceedings of the Royal Society B: Biological Sciences. She’s identified four distinctive types of wounds, suggesting that multiple species of mini-vamps hunted in Earth’s young oceans.
For Porter, it’s the timing of the attacks that’s especially significant. The vampires were on the prowl during an era when life on our planet was undergoing swift, dramatic change. After being stuck in an evolutionary rut for roughly a billion years, new species were finally beginning to appear.
Porter believes the driving force behind this event was the emergence of mini-vamps and other types of predators. They unleashed an evolutionary arms race that compelled life to diversify in order to survive the onslaught.
Some critters, for instance, developed hard, biomineral skeletal structures and armored hides, while others found safety in numbers by forming colonies that became primitive, multicellular organisms—the earliest precursors to the varied and complex life-forms on our planet today.
If Porter is correct, we owe our existence to the tiny vampires and their ilk, who ushered in an era when life on Earth went from bland to brutal.
The central characters in this story are eukaryotes—complex, single-celled organisms that were the ancestors of animal, plant, and fungal cells.
Eukaryotes first made their appearance around 1.8 billion years ago. Back then, though, they were minor players in Earth’s ocean ecosystem, where the dominant life-forms were simple, single-celled bacteria, called prokaryotes, which lacked a distinct nucleus and other structures typical of modern cells.
That was the status quo for a billion years, a period in our planet’s history that scientists refer to as the “Boring Billion.” The eukaryotes ate bacteria and reproduced, creating more eukaryotes that ate and reproduced, and so on across the millennia.
But eventually, things got interesting. “There's evidence that, starting around 800 million years ago, maybe a little bit before, that eukaryotes really are diversifying and becoming more important on the planet,” says Porter. “We see new innovations, such as biomineralization [hard skeletal structures] and multicellular forms.”
Scientists don’t know what jump-started evolution during this time. Some say rising oxygen levels in Earth’s oceans allowed for the emergence of more complex organisms, whose metabolisms required more energy. Recent studies, however, suggest that some animals require less oxygen to function than previously thought. And fossil evidence indicates that eukaryotic diversification began before the onset of oxygenation.
Porter believes that, while factors such as increased oxygen might have played a role, the pivotal change occurred when new species of eukaryotes began to develop a taste for other eukaryotes.
Images show distinct circular wounds on the body of a fossil creature that lived 780 to 740 million years ago. [PHOTOGRAPH BY SUSANNAH PORTER]
“It could have been a density issue,” she says. “If eukaryotes themselves are becoming more and more abundant, then it makes sense that somebody would evolve to eat them.”
It makes sense in the bigger picture, as well. “There's been some studies that predators are actually useful for maintaining diversity at the lower levels in the food web,” says Porter. “No one individual organism in a species can take over, because predators will crop them back.”
What’s more, predators can trigger an evolutionary arms race, as both hunter and prey develop new traits in a constant struggle to keep one step ahead of the other.
But how does anyone prove that miniature predators actually existed some 800 million years ago? The microfossil of a voracious eukaryote doesn’t exactly have the distinguishing characteristics of, say, a Tyrannosaurus rex or a saber-toothed cat.
One strategy is to infer the existence of predators by looking at the prey. Porter cites the fossil of a single-celled organism, found in the Yukon region of Alaska and Canada, that was surrounded by armor-type scales covered in spines. That would have been effective defensive gear against predators that fed by engulfing and digesting smaller eukaryotes.
And while microfossils don’t yield gnawed bones for paleobiologists to discover, scientists do have the next best thing: the desiccated corpses of eukaryotes whose squishy insides were digested by vampires.
The advantage of this hunting method is that the predators didn’t have to be larger than their prey. They most likely fed in the same way that some modern-day, single-celled vampires—known as vampyrellid amoebae—consume their food.
First, they partially engulf their victims and use enzymes to dissolve part of the cell wall. Next, they form a temporary crude arm, called a pseudopod, and extend it through the opening, where they pinch off chunks of cytoplasm and consume it.
The victims bear the puncture marks where the vamps penetrated their bodies. While examining the microfossils, Porter found circular wounds of various sizes. Some are half a micron in diameter, while others measure one or two microns, and some measure 30 microns across. She also discovered a fourth type of wound with a peculiar half-moon shape.
The varying dimensions of the wounds, Porter believes, reflect different species of tiny vampires.
“The strategy of the ones that make the larger holes seems to be that they used enzymes to just digest a ring,” she says. “And then they remove the whole circle, almost as if they’re lifting up a manhole cover out of the street.” The larger hole, she adds, would have made feeding easier.
Porter did consider the possibility that the puncture marks could have been naturally occurring pores in the organisms’ cell walls. But she concluded that was unlikely, since the holes were irregular in terms of their number and distribution on the fossilized corpses.
“They were also quite rare in any one species—maybe less than 10 percent of any of the specimens had them, and yet they were found in at least seven very different species,” she says. “That suggested they weren’t biological traits.”
Perhaps the most intriguing, indirect evidence for the rise of eukaryotic predators was the appearance of simple, multicellular organisms.
When confronted with a threat, there’s safety in numbers. Porter cites a lab experiment conducted nearly 20 years ago, in which a predator was introduced into a community of single-celled eukaryotes—a green algae species called Chlorella vulgaris.
Within less than a hundred generations in the lab, the Chlorella began coming together to form clusters, eventually settling on eight-celled colonies. It proved to be an ideal size: too large to be swallowed by the predator, but small enough that each of the individual colony members remained exposed to the surrounding nutrients in their glass-tubed environment.
What’s nifty about this behavior is that it can be an effective defense against various types of predators for different reasons. If you don’t want to be engulfed and digested, then the formation of multicellular colonies can make you too big to swallow.
And if you're dealing with the vampiric variety, then “you’re not going to be as bothered by this type of predation—in the same way that having a mosquito suck your blood doesn't kill you,” says Porter. Attacking one of the eukaryotes, in other words, won’t prove fatal to the colony as a whole.
These early, multicellular colonies enabled the evolution of more complex plants and animals, including us.
In essence, life as we know it may exist because of eukaryotes that, hundreds of millions of years ago, used their own bodies to create organic fortresses against marauding vampires.