Human DNA intact after 8,000 years.

Human DNA intact after 8,000 years

For nearly three years, scientists working at a central Florida peat bog have been dredging up 8,000-year-old human skeletons with skulls that contain shriveled, remarkably preserved brains (SN: 12/22 & 29/84, p. 388). They now report that four of the brains have yielded the oldest-known examples of human DNA and cellular structure.

Molecular biologists are now attempting to clone genes or gene fragments from the prehistoric pieces of DNA so that they can be compared to corresponding modern genes. The charting of clear-cut evolutionary "mutations' or changes in specific genes will likely require the examination of much older specimens, however.

Preservation of soft tissue in the soggy bog, say anthropologist Glen H. Doran of Florida State University in Tallahassee and his colleagues, demonstrates that "intact DNA can survive in other than extremely arid conditions, which greatly widens the sites where ancient genetic material may be found.'

Previous DNA recoveries from archaeological remains have involved dried tissue. One researcher detected humanlike fragments of DNA in 3 of 23 Egyptian mummies (SN: 4/27/85, p. 262) and cloned some of the 2,400-year-old DNA segments in bacteria. Bits of DNA from a quagga, an extinct horselike animal, have also been cloned (SN: 6/9/84, p. 356).

The yield of DNA from the Florida specimens was about 1 percent of the amount normally obtained from fresh tissue. DNA analysis is being conducted by Philip J. Laipis and William W. Hauswirth of the University of Florida College of Medicine in Gainesville.

It is difficult to isolate clean samples of the ancient genes, says Laipis, because of considerable damage to the DNA and the intrusion on plant residue and other impurities picked up from the soil. But rapid advances in genetic analysis are being made at several laboratories, he adds.

"The average size of the DNA strands we've isolated is small, only a few hundred base pairs per strand,' notes Laipis. "We're attempting to clone pieces with Alu fragments.' Alu repeated sequences are regions of DNA that are characteristic of human DNA.

It is not probable that genetic mutations have occurred in the short evolutionary time span of 8,000 years, says Laipis. But as cloning methods for damaged DNA improve, he and other investigators plan to compare genetic

material from much older specimens to that of their modern counterparts. For example, Laipis suggests that gene fragments from a frozen, 36,000-year-old steppe bison uncovered in Alaska could be matched with corresponding fragments from a modern buffalo.

In addition to the DNA findings, which are reported in the Oct. 30 NATURE, microscopic examination of samples taken from the cerebral hemispheres, cerebellum and brain stem revealed, according to the researchers, "limited but definite' remains of cell structure and patterning similar to that found in modern brains.

While the prehistoric brains keep molecular biologists busy, the peat bog where the specimens were preserved-- located in a suburban housing development that is now building around the site --is also proving to be an important resource for archaeologists. In October, a piece of fabric with a sophisticated weave was found wrapped around a skeleton, suggesting that these early Americans were able to develop skilled crafts. They also seem to have cared for severely handicapped individuals, as indicated by the skeleton of an adolescent boy with bone deformities that probably crippled him from birth.

In addition, says Doran, it is clear from the more than 40 skeletons found so far that the site was burial ground. The skeletons represent both sexes and a wide range of ages and now lie under about 7 feet of peat, which is covered by 2 to 3 feet of water.

Photo: Cross-sections of modern (left) and 8,000-year-old (right) brains. Despite damage, many anatomical structures survive in the prehistoric gray matter.