The Amazing Trace: Blood molecules survive for millions of years

This image is a microscope photograph of a piece of shale from the Kishenehn Formation in northwestern Montana containing the fossil of a blood-engorged mosquito. Scientists from the Smithsonian and the Natural History Museum in London have discovered biomolecules from the blood in the mosquito’s abdomen that have been preserved for 46 million years.

Sometime during the Middle Eocene a prehistoric mosquito slurped down a final blood meal, died and sank to the bottom of a pond in what is now northwestern Montana. Slowly covered in fine sediments at the bottom of the pond it eventually became encased and compressed in a protective layer of shale. Now, that mosquito and its blood-filled abdomen are providing scientists stunning new evidence that blood molecules can be preserved through deep time—in this case 46 million years.

Using a scanning electron microscope and mass spectrometry, a team of scientists led by Dale Greenwalt of the Department of Paleobiology at the Smithsonian’s National Museum of Natural History discovered iron and porphyrin molecules from the mosquito’s last supper very much intact inside the fossil. Porphyrin is a large planar molecule that binds iron and oxygen in the blood.

Other members of the research team included Yulia Goreva, Sandra Siljeström and Tim Rose of the Department of Mineral Sciences at the Smithsonian’s National Museum of Natural History, and Ralph Harbach of the Department Life Sciences of the Natural History Museum, London. Their paper on the discovery appeared in the Proceedings of the National Academy of Sciences on Oct. 14.

“This is the only known fossil of a blood-engorged mosquito ever found and represents the first clear evidence that some organic molecules can be preserved in a fossil of this age,” Greenwalt explains.

To detect the molecules the team first used a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer to identify various elements in the fossil. They located large amounts of iron specifically in the mosquito’s abdomen.

Smithsonian paleobiologist Dale Greenwalt holds a piece of shale from the Kishenehn Formation in northwestern Montana containing the fossil of a blood-engorged mosquito.  (Photo by James DiLoreto)

“Having found the iron we asked ourselves: Can we find the prophyrin molecule?” Greenwalt says. The team used a Time-of-Flight secondary ion mass spectrometer and “we readily found a very beautiful spectruma specific fingerprint—for the prophyrin molecule. The signal we obtained was very indicative of the presence of the two together—iron and prophyrin—one bound to the other in the unmistakable signature of blood.” This is the first time that this particular organic molecule has ever been definitively identified and localized in a fossil.

“We made the assumption that genetic material like DNA has not been preserved,” Greenwalt says. “We didn’t even attempt to look at it because DNA is a very liable molecule that degrades quickly.” Still, he says, this discovery opens a door to further exploration. “Without question there are probably other things contained in this fossil. We just don’t know what they might be.”

The Kishenehn Formation in northwestern Montana is unique in that it exhibits a spectacular preservation of very tiny insects like mosquitoes. Earlier this year Harbach and Greenwalt were able to describe and name two new species of mosquito, long since extinct, because their tiny body parts—wing veins, sexual organs, scales and hair-like structures on the wings—had been exquisitely preserved. Those mosquitoes were in the genus Culiseta which today feed mainly on birds, Greenwalt says. “But we have no way of knowing what the host for this blood-engorged mosquito was.”

“One of the other characteristics of insects from the Kishenehn Formation that is preserved quite often is color,” Greenwalt adds. “We have yellow insects and red insects and orange insects.”

What conditions permit such exquisite preservation? “We don’t know but we are studying it,” Greenwalt says. The Montana climate was much warmer then—wet subtropical to tropical. “You can imagine when you think of a blood engorged mosquito, the abdomen is just blown up like a balloon and it is very fragile. If it hits water, it hits land, it hits anything, the first thing that’s going to burst would be that abdomen. Obviously the conditions that allow for such preservation are very unique and very unusual.”

“This fossilized female mosquito is an incredibly rare find,” says co-author Harbach. “For it to have died immediately after feeding and be preserved without disruption to its fragile distended blood-filled belly means that we have a unique opportunity to study whether complex molecules, such as hemoglobin, can survive tens of millions of years.

“Our findings are a tantalizing glimpse into the past, not only helping us to better understand the evolution of blood-feeding in insects, but also opening up the possibility that other complex molecules, under the right conditions, might also be preserved through time.”

Known as haematophagy, blood-feeding occurs in roughly 14,000 insect species known today, including fleas and mosquitoes. Although this feeding strategy appears to have evolved independently across a variety of animals, fossil evidence of this behavior is extremely rare. This find extends the fossil record of blood-feeding to 46 million years.

Dale Greenwalt, left, and Tim Rose of the Department of Mineral Sciences at the Smithsonian’s National Museum of Natural History, use an energy-dispersive X-ray spectrometer to study the blood deposits in the fossil of a prehistoric mosquito. (Photo by James DiLoreto)

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