Scientists Calculate the Half-Life of DNA using Moa Fossils
A team of international researchers have used the fossilised bones of three species of extinct giant birds from New Zealand to calculate the half-life of DNA. This study suggests that the double helix can persist in the fossil record under optimum conditions for a lot longer than previously thought with traces of genetic material being detectable in fossils as old as 6.8 million years. The work, which is controversial, if proved valid, rules out DNA from a dinosaur surviving so there would be no chance of cloning a member of the Dinosauria from genetic material recovered from their bones or from biting insects trapped in amber.
Sorry Michael Crichton fans, but his wonderful idea about a dinosaur populated “Jurassic Park” is simply not on.
A half-life measurement records the time required for a substance to fall to half its measured value at the beginning of a time period. One of this term’s most common applications is in the measurement of radioactive decay. Within palaeontology for example, once a half-life of a substance such as elements from an igneous rock deposited in association with sedimentary strata is calculated, since the rate of decay is exponential, this methodology can permit scientists to accurately date rocks and potentially any fossil material associated with adjacent strata. However, a team of scientists have worked out a half-life for DNA itself. If this measuring technique proves valid then the dating of fossils could become a lot easier and the search for DNA samples within the fossil record can become more targeted.
There have been a number of papers published recently that claim to have isolated extremely old, fragmentary DNA, even elements of organic material from dinosaur bones. The need for a reliable model for DNA degradation over the passage of time has been well established. The international team of palaeontologists took core samples from the leg bones of 158 specimens of New Zealand Moas which were very likely to have preserved in them mitochondrial DNA. Radiocarbon dating allowed the team to accurate work out the ages of the fossil material and based on this analysis they were able to demonstrate that DNA decays at a exponential rate over time. The half life of DNA was calculated to be 521 years, much longer than had been demonstrated in other experiments.
Fossilised Leg Bones Used in the Study
Large Moa Bones used in the DNA study.
Picture Credit: Morten Allentoft
After an animal dies, the cells begin to degrade. Enzymes start to dissolve the bonds between the nucleotides that form the structure of the DNA material contained within the cell. Micro-organisms can speed up the decay process, but it is thought that the presence of ground water and the chemical reactions brought about by its presence, is mostly responsible for the degradation of the genetic material. As groundwater is abundant and found in most strata, so DNA buried in bone undergoing a fossilisation process should, in theory at least, degrade at a set, measurable rate.
Calculating the rate of DNA decay has been fraught with difficulties because of the problems of finding enough fossil material with large amounts of DNA with which to use in any scientific study. Compounding this problem is the fact that variable environmental conditions such as temperature, the amount of oxygen present and the level of microbial activity all have a significant impact on the decay of organic material.
The research team led by Morten Allentoft (University of Cophenhagen, Denmark) and Michael Bunce (Murdoch University, Perth, Australia) focused their efforts on analysing the DNA from 158 leg bones that belonged to three species of extinct Moa. Moas were giant, flightless birds (nine species) that were native to New Zealand (Dinornithiformes), some species were over 3.5 metres tall. These birds, closely related to Australian Emus, became extinct around 1400 AD. These creatures were once abundant on both North and South Island and the bones used in the study came from three locations all within a few miles of each other. The close proximity of the specimens studied enabled the scientists to nullify the effect of environmental differences between locations as the fossils had been forming in almost identical preservation conditions.
The Moa – Helping to Unlock the Half-life of DNA
Bones of Moas helping to unravel the half-life of DNA.
Picture Credit: Frans Lanting/National Geographic Stock
All the bones have been dated between 8,000 and 600 years old, the strata in which they were being preserved had a temperature of around thirteen degrees Celsius, helping to keep the results of any DNA half-life measurement consistent over the entire sample.
By comparing the specimens’ ages and degrees of DNA degradation, the researchers calculated that DNA has a half-life of 521 years. That means that after 521 years, half of the bonds between nucleotides in the DNA would have broken; after another 521 years half of the remaining bonds would have degraded leaving only a quarter of the original material left; and so on. Using their research, the team have postulated that detectable DNA could be found in fossils as old as 6.8 million years, but this material would be too fragmented to be used in any cloning work. DNA’s ability to survive in the fossil record, or so it seems, has been seriously underestimated.
Post doctoral researcher, Morten Allentoft commented:
“DNA degrades at a certain rate, and it therefore makes sense to talk about a half-life.”
These results may provide a baseline for predicting long-term DNA survival in fossil bone, helping palaeontologists to assess the most likely fossils to have sustainable amounts of DNA within them. In sub-zero conditions, such as those found in Siberia, DNA may have a half-life that it much longer, perhaps as much as 158,000 years. This would potentially permit scientists to extract viable DNA from Ice Age mammals such as Woolly Rhinos and Mammoths.
A number of scientists have yet to be convinced by these findings. Eva-Maria Geigl at the Jacques Monod Institute (Paris, France), remains sceptical. She is concerned that the analysis rests on statistically weak evidence, pointing out that the correlation relies heavily on the Moa bones older than 6000 years – when fewer than 10 of the 158 bones are actually as old as this.
Michael Bunce defended his work by explaining:
“Old fossils are rare and hence there will be less data in this part of the analysis. There is nothing we can do about it other than present what we have at hand – and clearly, the signal is present. The correlation is highly significant.”
If genetic material has a predictable time-frame for decay, then palaeontologists may have an opportunity to obtain DNA from important fossil discoveries that reveal life on Earth in the relatively recent geological past.
Ever since the Indonesian island of Flores yielded remains of a pygmy-like hominid (Homo floresiensis), nick-named the “hobbit” speculation has been rife that some specimens might contain DNA that would help pin down its position in the human family tree. Scientist remain uncertain whether these little people were descendants from modern humans or the much older H. erectus.
Unfortunately, exogenous factors would “cloud” the DNA half-life calculations. The conditions in which the fossils were preserved, the degree of groundwater, the amount of oxygen, the level of microbial activity and the ground temperature would all affect the rate of genetic decomposition. The research scientists conclude that “a host of other factors would come into play“, including the time of year when the organism died. Although the Moa bones used in the study had all been retrieved from very similar environments, the age of the specimens could only account for about 40% of the variation in DNA preservation. The research team admits that the “half-life signal is very noisy“.
How a corpse rots and a whole host of other factors would influence the rate of decline of any genetic material once present, based on this work retrievable and workable DNA could potentially be recovered from a fossil that was 1.8 million years old – but beyond this time-frame sufficient DNA recovery to permit effective study would be virtually impossible.
Looks like the non-avian dinosaurs are really extinct after all.