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DNA’s four nucleotides and find differences in the sequences between the DNAs of two individuals or two species. The more similar two sequences were—that is, the fewer the number of differences between them—the more closely related they were. In fact, from the number of shared mutations we could infer not only how the particular sequences had evolved from common ancestral DNA sequences over thousands and millions of years but also approximately when those ancestral DNA sequences had existed. For example, in a 1981 study the British molecular biologist Alec Jeffreys analyzed the DNA sequence of a gene that encodes a protein in the red pigment in the blood of both humans and apes and deduced when the genes began evolving independently in humans and apes. This, I explained, could soon be done for many genes, from many individuals of any species. In this way, scientists would be able to determine how different species were related to one another in the past, as well as when they began their separate histories, with much greater accuracy than was possible from the study of morphology or fossils.
    As I explained all this to Rosti, a question gradually arose in my mind. Would this kind of investigation necessarily be restricted to DNA from blood samples or tissues from humans and animals that live today? What about those Egyptian mummies? Could DNA molecules have survived in them—and could they, too, be joined to plasmids and made to replicate in bacteria? Could it be possible to study ancient DNA sequences and thereby clarify how ancient Egyptians were related to one another and to people today? If that could be done, then we could answer questions that no one could answer by the conventional means of Egyptology. For example, how are present-day Egyptians related to Egyptians who lived when the Pharaohs ruled, some 2,000 to 5,000 years ago? Did great political and cultural  changes, such as the conquest by Alexander the Great in the fourth century BCE, or by the Arabs in the seventh century AD, result in replacement of a large part of the Egyptian population? Alternatively, were these just military and political events that caused the native population to adopt new languages, new religions, and new ways of life? In essence, were the people who lived in Egypt today the same as those who built the pyramids, or had their ancestors mixed so much with invaders that modern Egyptians were now completely different from their country’s ancient population? Such questions were breathtaking. Surely they must have already occurred to someone else.
    I went to the university library and searched in journals and books but found no report of any isolation of DNA in ancient materials. No one seemed even to have tried to isolate ancient DNA. Or if they had, they had not succeeded, because if so, surely they would have published their findings. I talked to the more experienced graduate students and postdocs in Pettersson’s lab. Given how sensitive DNA is, they argued, why would you expect it to last for thousands of years? The conversations were discouraging, but I didn’t give up hope. In my forays into the literature, I had found articles whose authors claimed to have detected proteins in hundred-­year-old animal hides in museums—proteins that could still be detected by antibodies. I had also found studies claiming to have detected, under the microscope, the outlines of cells in ancient Egyptian mummies. So something did seem to survive, at least sometimes. I decided to do a few experiments.
    The first question seemed to be whether DNA could survive for long in tissues after death. I speculated that if the tissue became desiccated, as was the case when a mummy was prepared by the embalmers in ancient Egypt, then DNA might well survive for a long period since the enzymes that degrade DNA need water to be active. This would be the first thing to test. So in the summer of 1981, when not too many people were around in the lab, I went to