Marianne Dehasque and Love Dalen: In the world of science, successes are often highlighted while mishaps are swept under the rug. However, we chose to take a different approach.
This is the fascinating tale of how multiple generations of scientists collaborated to unravel the genome of the mammoth formerly known as Lonely Boy, the supposed last mammoth on Earth.
The woolly mammoth, a captivating species from the last Ice Age approximately 120,000 to 12,000 years ago, remains an enigma when it comes to its extinction. Mammoths once roamed vast regions of the northern hemisphere, but by the end of the Ice Age, they vanished from most of these areas. The final population of mammoths resided on Wrangel Island, located off the Siberian coast, until their ultimate extinction around 4,000 years ago.
Our recent study, featured in Cell, delved into whether the mammoth population on Wrangel Island was genetically doomed for extinction. Despite numerous setbacks, we eventually uncovered that this was not the case.
The mammoths became isolated on Wrangel Island roughly 10,000 years ago due to rising sea levels and persisted as a small population for millennia. Inbreeding, a common issue in small populations, could lead to detrimental effects over time, potentially resulting in an unsustainable population and eventual extinction.
Inbreeding poses a myriad of problems, as exemplified by Charles II of Spain, the last monarch of the Habsburg house, who displayed physical deformities due to generations of inbreeding. Scientists have long pondered whether genomic factors contributed to the extinction of the Wrangel Island mammoths.
To address this question, we compiled a dataset of 21 mammoth genomes spanning the last 50,000 years of the species’ existence. This dataset allowed us to journey back in time and examine the genetic consequences of their isolation.
Lonely Boy stood out as the most crucial individual in our dataset, serving as the key to unraveling the mysteries behind mammoth extinction. However, sequencing Lonely Boy‘s DNA proved to be a formidable challenge.
The Adventures of Lonely Boy
Creating a genome for Lonely Boy entailed multiple attempts over nearly a decade. Initially, our DNA extraction efforts were hindered by human contamination. Subsequently, we utilized bleach to eliminate as much contamination as possible.
While this practice is common in ancient DNA research, it also carries the risk of inadvertently destroying mammoth DNA along with contaminants. In our case, insufficient mammoth material remained in the sample to generate a high-quality genome.
In a final endeavor, we merged data from various DNA extracts from Lonely Boy. However, the DNA from these extracts, although similar, appeared to belong to distinct individuals, sparking wild theories to explain these puzzling results.
One prevailing theory suggested that Lonely Boy might have experienced the “vanishing twin syndrome,” where a mammal foetus absorbs the genetic material of a deceased twin during gestation. While this could explain the similarities in DNA extracts, it did not account for the discrepancies.
The true explanation turned out to be far less sensational, tracing back to the immense effort invested in the sample. This effort resulted in peculiar lab artefacts, which introduced false genetic variations. Consequently, we developed a filtering method to eliminate these artefacts.
Despite these measures, Lonely Boy remained an outlier. Consequently, we decided to re-date the sample, revealing that Lonely Boy was not 4,000 years old as initially assumed, but approximately 5,500 years old, making him an average mammoth in our dataset.
Causes of Extinction
Contrary to our initial hypothesis, the Wrangel Island mammoth population likely did not go extinct due to inbreeding. Our genomic data, coupled with computer simulations, indicated a significant population decline post-isolation on Wrangel Island, with only eight breeding individuals remaining.
However, the mammoth population swiftly rebounded to a size of 300 within 20 generations and remained stable until their extinction. This stability is evident in the consistent inbreeding levels throughout this period.
Yet, our findings on harmful mutations paint a different picture. While the most detrimental mutations were gradually eliminated through natural selection, mildly harmful mutations accumulated over time, hinting at enduring genetic effects from the initial population decline.
Predicting the precise impact of harmful mutations is challenging, particularly in an extinct species. Analogies with known human diseases suggest that purged mutations may have disrupted genes crucial for sensory development.
Nonetheless, these mutations likely did not contribute to the mammoth’s extinction. Based on our results, the extinction event was sudden, possibly triggered by an unforeseen occurrence such as a disease outbreak or severe weather event.
As with any scientific endeavor, further research may shed new light on the subject, perhaps uncovering another Lonely Boy or Lonely Girl.
Although our journey to analyze mammoth inbreeding was fraught with challenges and diversions, our lab group gleaned valuable insights from the project.
We devised a novel bioinformatics method to address human contamination and identified a new type of lab artefact. This newfound knowledge could prove pivotal in pinpointing the precise reasons behind the woolly mammoth’s extinction.
Marianne Dehasque, Postdoctoral Researcher, Uppsala University and Love Dalen, Professor, Stockholm University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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