One of the enduring mysteries in human evolution, contemplated since Charles Darwin's time, has recently offered a fresh insight: the timeline of when we lost our tails. Unlike many other primates such as lemurs and monkeys, apes – encompassing humans and chimpanzees – do not possess prominent tails. Advances in gene-editing technology have enabled researchers to unveil a captivating development in the narrative of ape tail loss. A recent study, disclosed in the journal Nature on February 28th, highlights a pivotal genetic transformation in our ape ancestors approximately 25 million years ago, offering potential insights into the evolution of our taillessness.
The quest for taillessness commenced approximately 20-25 million years ago, marking the divergence of our lineage from Old World monkeys. In this evolutionary split, the ancestors of apes underwent a shift, favoring a reduction in the number of tail vertebrae. This ultimately resulted in the formation of our coccyx, commonly referred to as the tailbone.
Although the precise reason for the loss of ape tails remains a matter of debate, intriguing theories abound. Some scientists propose that the abandonment of the tail could have conferred advantages to our ancestors as they transitioned to a more upright posture and increased ground activity. Tailed primates frequently rely on their tails for balance and movement, particularly when navigating tree branches horizontally through swinging and walking. This elucidates why tailless apes such as gibbons and orangutans, while still arboreal, display distinct movement patterns compared to their tailed monkey counterparts, who often dangle from branches.
Prior research has pinpointed more than 100 genes associated with tail development in diverse vertebrates. This has led scientists to posit that the absence of tails in apes probably stems from alterations in the DNA code, particularly through mutations in multiple genes, playing a role in this evolutionary transformation.
Jumping genes
In a groundbreaking study featured in the journal Nature, researchers delved deeper into the enigma of ape tail loss. They conducted a comparative analysis of the DNA from six ape species, including humans, and 15 species of monkeys. This thorough examination unveiled a pivotal aspect of the puzzle: a specific DNA insertion shared by apes and humans but not present in monkeys. This insertion took place within a gene called TBXT, recognized for its involvement in tail development across various animal species.
To delve deeper into the ramifications of this mutation, researchers utilized CRISPR technology. In a laboratory context, they edited the TBXT gene in mouse embryos, mimicking the observed mutation location in apes and humans. The outcomes were remarkable: mice with modified TBXT genes exhibited various tail abnormalities, with some individuals being born completely devoid of tails.
Nevertheless, the narrative doesn't conclude at that point. Researchers unearthed an additional layer of complexity. Although the TBXT mutation unquestionably contributes to the taillessness phenotype, DNA is a intricate system. Picture it as a twisted ladder, where each rung signifies a distinct gene. Alterations or mutations can transpire on individual rungs (single genes). However, more intricate modifications can involve multiple rungs simultaneously.
Introducing Alu elements: repetitive DNA sequences capable of transforming into RNA and then reverting to DNA. These "jumping genes" possess the capacity to insert themselves randomly into the genome, potentially either disrupting or enhancing the function of a gene based on where they land. In this scenario, the proximity of Alu elements to the TBXT mutation in certain individuals might have additionally impacted the outcomes of tail development in the mice. This underscores the intricate interplay of various factors in shaping complex biological phenomena, such as the loss of tails in apes.
The research team uncovered a captivating development in the narrative of ape tail loss. They identified two Alu elements within the TBXT gene—essentially "jumping genes" capable of relocating within the genome and potentially interfering with gene function. The crucial revelation is that these Alu elements were consistently present in all the great apes examined but entirely absent in monkeys. This implies that these distinct jumping genes, exclusive to primates, could have played a role in the genetic divergence that resulted in tail loss in our ape lineage millions of years ago.
Genetic trade-off?
The researchers recognize that any potential advantage gained from losing our tails must have been exceptionally significant. Genes frequently influence multiple functions, and a modification that offers an advantage in one aspect can sometimes yield negative consequences elsewhere. Intriguingly, the team noted a slight increase in neural tube defects in mice with the modified TBXT gene. This observation sparks a compelling theory for future exploration: could the ancestral loss of tails be associated with the occurrence of neural tube birth defects, such as spina bifida, affecting approximately one in every thousand human babies? Further studies will be crucial in exploring this potential evolutionary trade-off.