Evolutionary history is now set for an upheaval

The world is celebrating 150 years of the publication of The Origin of Species, the seminal work on evolution by Charles Darwin. But the study of evolution itself is constantly evolving as it has weathered many a stormy challenge, shedding several shibboleths in the process.

Evolutionary history, a crucial discipline in evolutionary studies, is now set for an upheaval, thanks to an analysis by researchers at Bangalore-based National Centre for Biological Sciences (NCBS) and Stanford University. The study says that ancient DNA, one of the crucial methods used to reconstruct evolutionary history, could have got it all wrong.

The first-time study, while reviewing the existing methodologies of reconstruction of the history of evolution, analysed 29 ancient DNA studies, summarised the analytical methods and general conclusions for each study and listed out the shortcomings of these methods.

One such ancient DNA conclusion on evolutionary history that is in peril is the belief that Indian rhinoceroses existing today are different from their ancestors. The divergence began 27,000 years ago, Swedish scientists had said after studying the ancient DNA of the rhino remains.

“The evolutionary history of a population involves changes in size, movements and selection pressures through time. Reconstruction of population history based on modern genetic data tends to be biased by reflecting only on recent events, leaving many population historic processes go undetected,” said one of the authors of the study, Uma Ramakrishnan.

In the re-analyses study, the researchers pointed out the need for new methods that consider the evolution of gene through generations (gene flow), multiple populations and population size in reconstruction of the history of population.

While fossil-based studies presented slow rate of mutations over a period of time, short-term pedigree-based methods showed high mutation rate. The problem with the study was the sample.

“The samples from different timeline, say one 10,000 years’ old and the another 100 years’ old from different parts of the world were considered for the analysis and hence came out with higher mutation rate. The problem here was that the migration, climatic conditions and geographical atmosphere were not considered and thus did account for complete reconstruction of the history of evolution,” Ramakrishnan said.

Ancient genetic data from multiple populations and multiple points in time and their analyses allow a direct view of the past and can yield powerful insights into the timing and magnitude of events in the history, the study said.

“Although it is theoretically, statistically, and empirically proven that ancient DNA data provide a better ability to investigate demographic history, the statistical analyses of ancient DNA data remain a challenge,” she said.

The biggest drawback is that the chronological aspect of sampling is not being considered explicitly. Also ecological studies have revealed that populations respond to climatic warming through dispersal to higher latitudes suggesting that gene flow could have been an equally important response to past climatic change, which is not considered in the method thus leaving it incomplete, she said.