"Why did all the giant mammals die out?" asked biologist Hendrik Poinar. For an answer, he reached into a storage cabinet in his lab and brought out a perfectly preserved clump of ancient animal poop. Yuck!
A giant ground sloth deposited the dung in a desert cave near Las Vegas about 20,000 years ago. The dry, sandy chunk of fossilized dung is called a coprolite or paleofeces. It is so perfectly preserved, you can see thin sticks of partially digested plants embedded in it.
Poinar says that genetic secrets buried in the poop could help explain why the elephant-sized ground sloth and more than 50 other species of giant mammals became extinct about 11,000 years ago.
THE SCOOP ON POOP
Poinar is a professor of genetics at McMaster University in Hamilton, Ontario. He is also the world's top poop sleuth. In 1998, he led a team of scientists that discovered how to extract deoxyribonucleic acid (DNA) from the fossilized dung of prehistoric creatures. DNA is a long molecule, present in every cell, that determines how an organism looks and functions. Every organism has unique DNA.
DNA extracted from ancient poop reveals lost secrets about how extinct mammals and prehistoric humans ate and lived. Old poop, says Poinar, is actually a richer source of DNA than are old bones. "A bone gives you the DNA from one animal, but poop gives you the DNA of the pooper and the animals and plants the pooper ate," he said as he showed off coprolite specimens from extinct ground sloths, saber-toothed tigers, and woolly mammoths in his lab, the "poop factory."
The key to the preservation of DNA in ancient dung, says Poinar, is the Maillard reaction. That is a chemical reaction that occurs when poop dries--or when sugar is used to bake cookies. As poop loses water and dries out, sugars from the plant material in the poop warm up and form sugar compounds that trap DNA molecules in a "sugar sandwich." "The DNA is frozen in time in a state of suspended animation," says Poinar.
Poinar extracts DNA trapped in the sugar sandwiches of old feces by using a new diabetes drug, PTB. High blood sugar is a symptom of diabetes, and the blood vessels of people who have the disease are often clogged with sugar compounds. PTB breaks down the sugar compounds and cleans out the arteries, enabling blood to flow more freely again. "The diabetes drug broke the sugar sandwich wide open," he says.
Poinar has analyzed DNA from three ground sloth dung samples found in Gypsum Cave in Nevada to reconstruct the lost world of the giant vegetarians. One sample revealed that ground sloths that inhabited the cave about 28,500 years ago fed on pine nuts and mulberries in a dry, temperate climate. Sloths that lived in the cave about 20,000 years ago feasted on capers, mustards, yucca, agave, grasses, mints, and possibly wild grapes in a somewhat cooler and wetter climate.
About 11,000 years ago, sloths subsisted on saltbushes (common desert shrubs) and had to roam longer distances to find water. This third sample, deposited at the end of the last ice age, suggests that conditions were hotter and drier then and vegetation was sparse. Might a gradual disappearance of vegetation have been one reason for the demise of the giant ground sloth and other giant mammals at that time? asks Poinar. Further dung samples may provide more clues.
Old poop can reveal a lot about prehistoric humans too. Many scientists believe that early Native American hunter-gatherers scrounged for berries and ate poorly. But DNA extracted from the 2,000-year-old paleofeces of three Native Americans discovered in a cave in Texas tells a different story. Poinar found that over a two-day period, the three natives feasted on a rich and diverse diet of pronghorn antelope, bighorn sheep, cottontail rabbit, pack rat, squirrel, fish, and eight different plants. Early Native Americans ate nutritious, high-fiber meals that were more healthful and better balanced than the processed foods many people eat today, he says.
Poinar hopes to solve one of the big mysteries of human evolution by extracting DNA from dung deposited by Neanderthals in caves in southern Europe about 30,000 years ago. Neanderthals were a short, stocky species of primitive human who became extinct at about that time. Poinar plans to search the DNA for a gene called FOXP2. A gene is a segment of DNA that determines a particular trait. FOXP2 is associated with human speech, and Poinar will look for it to determine whether Neanderthals had the ability to converse.
He also wants to compare Cro-Magnon and Neanderthal poop samples found in Israel to see whether Cro-Magnon DNA contains bits of Neanderthal DNA. Cro-Magnons were early modern humans who lived mainly in Europe from about 40,000 to 10,000 years ago. Finding Neanderthal DNA in Cro-Magnon DNA would suggest that the two groups interbred.
"It's exciting," says Poinar, "that ancient dung could help answer important questions about the early evolution of humans."
POOPER SCOOPER How to Extract DNA From Poop
1 Freeze a poop sample in liquid nitrogen; then grind it into a fine powder. The grinding won't destroy the DNA, which is preserved in fragments much finer than the powder.
2 Add the diabetes drug PTB to release the DNA trapped in "sugar sandwiches" in the poop powder.
3 Make millions of copies of the DNA in a polymerase chain reaction machine.
4 Determine the DNA's sequence, the long series of letters that represents the structure of the DNA molecule. The DNA sequence contains the information needed to identify the genetic remains of the human, animal, or plant in the poop.
5 Compare DNA sequences extracted from the poop with known DNA sequences for comparison. For example, match DNA sequences preserved in poop with DNA sequences from ground sloth bones found in Gypsum Cave.
Choose the response that best completes the statement or answers the question. Write the letter of the response in the blank provided.
--1. A piece of fossilized poop is also called (A) a coprolite. (B) paleofeces. (C) both A and B.
--2. In a Maillard reaction, which kinds of molecules form compounds? (A) sugars, (B) fats, (C) alcohols
--3. Scientists believe Neanderthals became extinct (A) 2,000 years ago. (B) 11,000 years ago. (C) 30,000 years ago.
--4. Which disease can have high blood sugar as a symptom? (A) tuberculosis, (B) diabetes, (C) leprosy
--5. A segment of DNA that determines a trait is called (A) a gene. (B) a promoter. (C) an operon.
--6. Early-modern humans who lived about 40,000 to 10,000 years ago are called (A) australopithecine. (B) paranthropine. (C) Cro-Magnon.
--7. The extinct giant ground sloth was as big as (A) a dog. (B) an elephant. (C) a whale.
--8. When did the last ice age end? (A) 28,500 years ago, (B) 20,000 years ago, (C) 11,000 years ago
--9. Which of the following animals is not extinct? (A) saber-toothed tiger, (B) komodo dragon, (C) woolly mammoth
--10. What does DNA stand for? (A) deoxyribonucleic acid, (B) dead northern animals, (C) dried Neanderthal
1. C 2. A 3. C 4. B 5. A 6. C 7. B 8. C 9. B 10. A