Ancient DNA from Extinct Animals

Apart from the occasional mummy or mammoth, DNA sequences from still living creatures are normally used to construct evolutionary schemes. However, ancient DNA extracted from the fossilized remains of extinct creatures can provide a valuable check on estimated evolutionary rates. The oldest available DNA so far successfully analyzed comes from amber. Amber is a polymerized, hardened resin produced by extinct trees that has gradually solidified to a glassy consistency over millions of years. Sometimes small animals were stuck in the resin when it oozed out of the trees and have been preserved there ever since (Fig. 20.27). Most of the trapped animals are insects, but occasionally worms, snails, and even small lizards have also been found. Amber acts as a preservative and the internal structure of individual cells from trapped insects can still be seen with an electron microscope. It has proven possible to recover DNA that is 25 to 125 million years old from some insects and some of this has been amplified by PCR and sequenced.

The largest chunks of amber are no more than 6 inches across, so larger animals such as dinosaurs cannot be preserved. Nonetheless, a few blood cells preserved in the gut of a blood-sucking insect could, in theory, provide the complete DNA sequence of a large animal. This was the scenario for Michael Crichton's high-tech thriller, Jurassic Park, where dinosaurs were resurrected by having their DNA inserted into amphibian eggs. In real life, such ancient dinosaur DNA would be severely damaged and only short segments would be readable. Nonetheless, the possibility of someday obtaining a few short fragments of some Tyrannosaurus rex genes is no longer a total fantasy.

Although DNA has indeed been isolated from samples that are several million years old it is so severely degraded that identification has not been possible. To date, the oldest identified animal DNA is approximately 50,000 years old and comes from mammoths preserved in the permafrost of Siberia. The permafrost has also yielded identifiable plant DNA from grasses and shrubs around 300,000-400,000 years old.

Microorganisms may also be trapped in amber and in such cases it may be possible to revive the whole organism, not merely obtain DNA samples. In particular,

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Genghis Khan's Y Chromosome

Large-scale surveys have shown that about 1 in 12 men in Asia carry a variant of the Y chromosome that originated in Mongolia roughly 1,000 years ago. Around 30 natural genetic markers were surveyed in several thousand men. The markers included deletions and insertions, sequence polymorphisms and repetitive sequences. Most men carry Y chromosomes with more or less unique combinations of such DNA markers. However, about 8% of Asian males carry Y chromosomes with the same (or almost the same) combination of genetic markers. This phenomenon was not seen among men from other continents. Furthermore, the Asian men with the special "Mongol cluster" of genetic markers were found only among those populations who formed part of the Mongol Empire of Genghis Khan. For example, the "Mongol cluster" was absent from Japan and southern China, which were not incorporated into the Mongolian Empire, but was present in 15 different populations throughout the area of Mongolian domination (Fig. 20.26). In addition, although very few Pakistanis have the "Mongol cluster", about 30% of a small tribal group known as the Hazara do possess it. The Hazara are known to be of Mongolian origin and claim to be direct descendents of Genghis Khan. Since present-day Pakistan is outside Genghis Khan's area of conquest they presumably migrated to their present location later.

This particular variant has therefore been proposed to be the Y chromosome of Genghis Khan the great Mongolian conqueror. About 800 years ago the warlord Temujin united the Mongols and in 1206 assumed the title of Genghis Khan ("Lord of Lords").The Mongols massacred many of the males and impregnated many of the women in areas they conquered. The present day distribution of Y chromosomes apparently reflects these practices. Whether this special variant of the Y chromosome was present in Genghis Khan himself or just frequent among his Mongol warriors cannot be known for certain. Nonetheless, it is more likely than not that Genghis Khan himself had this Y chromosome, as all the warriors in such tribes were usually closely related.

FIGURE 20.26 Genghis Khan's Empire and Y Chromosome

The relative proportion of Mongol cluster chromosomes at various geographical locations is represented by the green segments in the circles. The size of the circles indicates sample size. From Zerjal and Tyler-Smith, The genetic legacy of the Mongols, American Journal of Human Genetics (2003) 72:717-721.

FIGURE 20.26 Genghis Khan's Empire and Y Chromosome

The relative proportion of Mongol cluster chromosomes at various geographical locations is represented by the green segments in the circles. The size of the circles indicates sample size. From Zerjal and Tyler-Smith, The genetic legacy of the Mongols, American Journal of Human Genetics (2003) 72:717-721.

FIGURE 20.27 Ancient DNA Preserved in Amber

At some point millions of years ago, a bee was trapped in sap from a tree. The sap gradually hardened and solidified into solid clear yellowish material—amber. The bee was completely preserved together with the bacterial spores that it carried. After extraction from the amber resin, occasional spores are capable of growth given the right nutrients and environmental conditions.

FIGURE 20.27 Ancient DNA Preserved in Amber

At some point millions of years ago, a bee was trapped in sap from a tree. The sap gradually hardened and solidified into solid clear yellowish material—amber. The bee was completely preserved together with the bacterial spores that it carried. After extraction from the amber resin, occasional spores are capable of growth given the right nutrients and environmental conditions.

spores, covered by a protective coat are formed by some bacteria to survive bad conditions and may survive for extremely long periods. Some 30 million-year-old bacterial spores have been found inside bees trapped in pieces of amber.When provided with nutrients, the spores grew into bacterial colonies. These reawakened bacteria were identified as Bacillus sphaericus, which is found today in association with bees. DNA from the ancient Bacillus sphaericus was similar in sequence to its modern relative, but not identical, as it would have been if the ancient bacteria were just contaminants. More recently, spores of another bacillus species were isolated and revived from a brine inclusion within a 250-million year old salt crystal.

Genetic information may be passed "vertically" from an organism to its direct descendents or "horizontally" to other organisms that are not descendents.

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