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.”
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amoeba A single-celled microbe that catches food and moves about by extending fingerlike projections of a colorless material called protoplasm. Amoebas are either free-living in damp environments or they are parasites.
cell The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size.
diversity (in biology) A range of different life forms.
enzymes Molecules made by living things to speed up chemical reactions.
eukaryote Any organism whose cells have a nucleus. Eukaryotes include all multicellular creatures (such as plants, animals and fungi) as well as certain types of single-celled microorganisms.
evolution A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.
fossil Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.
geology The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists. Planetary geology is the science of studying the same things about other planets.
microbe Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.
mineral The crystal-forming substances, such as quartz, apatite, or various carbonates, that make up rock. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in certain regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
paleontologist A scientist who specializes in studying fossils, the remains of ancient organisms.
predation A term used in biology and ecology to describe a biological interaction where one organism (the predator) hunts and kills another (the prey) for food.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
scanning electron microscope (SEM) A scientific instrument in which the surface of a specimen is scanned by a beam of electrons that are reflected to form an image.