A freak DNA change 25 million years ago is why humans lack tails
- April 9, 2024
- Posted by: OptimizeIAS Team
- Category: DPN Topics
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A freak DNA change 25 million years ago is why humans lack tails
Subject: Science and Tech
Section: Biotech
Introduction:
- A distinguishing anatomical trait of apes, setting them apart from monkeys, is their lack of a tail. While all mammals possess a tail at some stage of development, apes—including humans, chimpanzees, bonobos, gorillas, orangutans, and gibbons—shed theirs during fetal development, leaving only a few vestigial vertebrae known as the coccyx or tailbone.
- This evolutionary change occurred approximately 25 million years ago when apes diverged from a common ancestor shared with monkeys.
The compact genome:
- Every cell within an organism houses a complete set of the organism’s DNA, termed the genome, which encodes the instructions for making proteins—key functional components of the cell. Specific segments of the genome, known as genes, are responsible for coding individual proteins.
- Cells selectively produce proteins. This selective production is achieved by creating a temporary copy of the gene in the form of messenger RNA (mRNA), which then guides the synthesis of the corresponding protein.
‘Junk’ DNA:
- In complex organisms like humans, genes are significantly spread out across the genome, with only about 1.5% of the human genome actually coding for proteins. The vast majority of the genome, previously deemed ‘junk’ DNA due to its unclear purpose, is now understood to play crucial roles, including regulating gene expression—essentially controlling when and how proteins are made.
- A notable portion of this ‘junk’ DNA comprises transposable elements, which are segments of DNA capable of moving and replicating themselves within the genome.
- One specific transposable element, known as Alu and exclusive to primates, is relatively small at about 300 base pairs but is extraordinarily prevalent, with approximately 1.4 million copies scattered throughout the human genome.
- These elements generally transpose within the genome without significant effects on health or evolution since their insertion impacts only the cell where the event occurs. For instance, if an Alu element inserts itself into a critical gene in one cell, only that cell may be adversely affected, leaving surrounding cells unaffected. However, if such an insertion occurs in the zygote—the initial cell formed at conception—this alteration in the DNA becomes permanent and is replicated in every cell of the resulting offspring, potentially having far-reaching consequences.
The Alu accident:
- Approximately 25 million years ago, following the divergence of ape and monkey ancestors, a rare event occurred: an Alu element inserted itself into a crucial gene within the zygote of an ancestral creature. This extremely unlikely insertion, with odds of about one in a million, led to a significant evolutionary trait—the absence of a tail in that creature and all its descendants, marking the lineage of all modern apes.
- This discovery was reported by scientists from New York University (NYU) in a Nature paper published in February. The research team embarked on a meticulous investigation, examining 31 genes known to influence tail development across apes and monkeys. Through this comparative study, they identified tens of thousands of mutations, deletions, and insertions that might have contributed to the loss of the tail in apes. However, none of these genetic changes, located within the protein-coding regions of DNA, definitively explained the phenomenon.
- The crucial Alu element was eventually found within the so-called ‘junk’ DNA, a part of the genome not involved in coding for proteins but known to contain elements regulating various genetic functions. This discovery underscored the importance of ‘junk’ DNA in evolution and the development of distinct traits in organisms.
A tailoring defect:
- In complex animals, genes are not continuous but segmented within the genome, interspersed with ‘junk’ DNA, and assembled only during mRNA creation. This structure allows for the versatile generation of different proteins from the same genetic sequence. Researchers from New York University (NYU) identified an Alu element insertion in the TBXT gene, crucial for tail development, which disrupts this gene’s proper assembly in apes, leading to the production of a defective TBXT protein and, subsequently, tail loss.
- This groundbreaking discovery was further confirmed by comparing TBXT mRNA in human and mouse stem cells, revealing defects in the human mRNA as anticipated.
- Beyond tail loss, the defective TBXT protein also led to neural tube defects, suggesting that compensatory genetic changes have occurred to mitigate these adverse effects.
Source: TH