Evolution Of Human Hand Dexterity

Some of us are really good with our hands.

But even if you can't do "David Blaine"-style card tricks with your fingers, you'd still have greater hand dexterity than our closest primate relatives - the chimpanzees.

Quick hands and smart brainz are hallmarks of the human species.

However, 99% of the DNA sequence in chimpanzees and human beings are identical, which means that the genetic sources for uniquely human characteristics are difficult to find.

Recently, biologists from the USA and Singapore have uncovered a region of the human genome that potentially plays a key role in the evolution of human hand dexterity.

While comparing the DNA sequences of numerous vertebrate species, they found a highly conserved element which is 546 base pairs long. Within this element there are 16 base changes that are unique to human beings and not found in the other vertebrates, not even chimpanzees.

The researchers called this element the "human-accelerated conserved noncoding sequence 1" (HACNS1), because the 16 base changes have evolved rapidly since the human-chimpanzee split (~6 million years ago) and also because it does not code for proteins.

To test if these changes have any effect at the animal level, they inserted equivalent elements of the human, chimpanzee and rhesus monkey into transgenic mouse embryos and compared the expression pattern:



















They found that only the human HACNS1 is strongly expressed in the anterior (front) limb bud of both the forelimbs (corresponding to arms in primates) and hindlimbs (legs).

The equivalent element in chimpanzee and rhesus monkey could only produce weak expression at the base of the limbs (shoulders).

Thus HACNS1 has a unique pattern of expression.

















I think this photograph is particularly striking - it shows the details of the forelimb in an older embryo.

The arrow points to the most anterior digit, which corresponds to the thumb in primates.

This is an important finding because the human thumb is rotated more towards the palm compared to other primates. Since HACNS1 is expressed here, it could be the genetic reason why the human thumb develops differently from other primates.

It's incredible to think that such large effects result from such a small number of DNA base changes. In fact, further experiments have shown that changing 13 of the 16 base pairs would already be enough to produce this effect.

To be sure, this research study cannot conclude that HACNS1 plays a direct role in the evolution of hand dexterity, because the effects were examined in mice rather than in primates.

In addition, the exact molecular mechanism that mediates human-specific hand development is not yet known. The researchers postulate that HACNS1 functions as an "enhancer" that can change the expression level of nearby genes such as CENTG2 and GBX2.

Even so, this study provides a tantalizing glimpse into the genetic details of human evolution.

Why, without those crucial 13 base pairs, I might not be able to get my hands on a big, round and succulent...















...fruit.


Would you like to know more?

- Original paper: Human-Specific Gain of Function in a Developmental Enhancer (Prabhakar et al. 2008, Science)
- Junk DNA may have handed us a gripping future(New Scientist)