The question of how humans and apes lost their tails has confounded scientists for generations. Now, a recent study, published on February 28th in the prestigious journal Nature, has revealed a genetic breakthrough explaining this long-standing mystery.
By comparing the DNA of tail-less apes and humans with tailed monkeys, researchers identified a specific DNA insertion that played a pivotal role in the evolutionary loss of tails in primates.
The investigation centered around a gene known as TBXT, which had been previously linked to tail development in vertebrates. More than 100 genes had been associated with tail growth, but this latest study found that it wasn’t mutations within TBXT itself that were responsible.
Instead, it was the insertion of a particular DNA segment, known as AluY, into the gene’s regulatory code that had the profound effect.
AluY belongs to a group of genetic elements referred to as “jumping genes” or retrotransposons, which can insert themselves into various places within the genome.
While typically dismissed as ‘junk’, these sequences have shown to play roles in genetic diversity and evolution.
The research team, led by then-student Bo Xia, demonstrated that this AluY insertion within TBXT affected the gene’s RNA splicing, leading to the production of different forms of TBXT RNA and, consequently, a variety of tail lengths in mice engineered to carry the mutation.
“This finding is remarkable because most human introns carry copies of repetitive, jumping DNAs without any effect on gene expression, but this particular AluY insertion did something as obvious as determine tail length,” explained study co-author Jef D. Boeke.
The research also showed that the AluY element is present in great apes, including humans, but is absent in monkeys.
Notably, the study suggests a profound evolutionary advantage for tail loss, potentially linked to bipedalism and life on the ground, despite the possibility of detrimental side effects.
Indeed, the insertion was associated with a slight increase in neural tube defects in mice, reminiscent of conditions like spina bifida seen in human neonates.
The research team, which includes experts from NYU Grossman School of Medicine and the Broad Institute of MIT and Harvard, hopes to further explore the theory that tail loss in our ancestors may have contributed to the occurrence of these neural tube defects.
The insights from this study pave the way for a deeper understanding of the genetic basis of tail loss in humans and apes and could influence broader genomic analysis in the future.
This breakthrough not only illustrates the intricate ways in which genetic elements can drive evolutionary changes but also underscores the complexity and adaptability of the human genome.
It highlights the shifting perspective on ‘junk DNA’ from genomic clutter to key evolutionary players, shaping our understanding of the processes that have crafted the human species as we know it today.