Nail is a hard covering of all finger and toe tips. They may be altered due to external or internal (systemic) factors. The nails in humans are considered as an aesthetic outgrowth, but its real function is to protect the soft ends of the phalanges or digits. They help to pick up small objects and are an important tool for scratching. In other species, this structure is slightly different, for example, crocodiles have a thimble-shaped structure that covers the whole tip of their digits, whereas hawks and owls have talons, which are highly curved claws specialized for prey capture, and horses and cows have hooves that protect their feet (Figures 1.1A to C). Comparative analysis of nail, claw, and hoof morphogenesis reveals relatively subtle differences in mesenchymal and epithelial pattern underlying these adult differences in distal limb appendage morphology.1
Nails have evolved from claws of reptiles. Primitive snakes like pythons and boas have vestigial hind limbs, which are tiny, clawed digits known as anal spurs. Claws are typically curved ventrally (downwards in animals) and compressed sideways. They serve a multitude of functions including climbing, digging, and fighting and have undergone numerous adaptive changes in different animal taxa. Claws are useful on large-diameter branches, whereas wide fingertips with nails and epidermal ridges are required for habitual locomotion on small-diameter branches. With the evolution of grasping hands and feet in primates, claws were no longer necessary for locomotion, and instead a reduction in the thickness of deep layer of the claw led to the advent of nails.2
Figures 1.1A to C: (A) Thimble-shaped structure covering the digit of crocodile; (B) Talons in birds; (C) Hoof
A study predicted morphoclines in fingertip morphology among four small-bodied (<1 kg) New World monkeys in order to test previous functional and adaptive explanations for the evolution of flattened nails, expanded apical pads, and grasping extremities within the order primates. The observed morphoclines demonstrate that a gradient in form from claw to nail like tegulae exists among these taxa. Thus, the distinction between claw and nail bearing platyrrhines is essentially arbitrary. According to Cartmill's (1972) functional and adaptive model for the loss of claws in primates, expanded apical pads are required for habitual locomotor and postural behaviors on small-diameter supports whereas claws are more useful for positional behaviors on large-diameter substrates.3
A nail is homologous to a claw but is flatter and has a curved edge instead of a point (Figure 1.2). A nail that is big enough to bear weight is called a ‘hoof’. Some marsupials also have nails and this is a good example of convergent evolution. It shows that flat nails are an important goal for evolution. Appearance of nails led to the development of critical functions, including finger pads that allow for sensitive touch and the ability to grasp.
The issue of whether nails or claws were present on the digits of the last common ancestor of living primates is central to the understanding of the ecological context in which the order originated. The combined new and old data indicate that the last common ancestor of the extinct primates had lost the typical mammalian claws of its ancestors and developed nails on all pedal digits except digit II, which bore a toilet-claw.4 Toilet claw is an intermediate evolutionary step between claws and nails. Their shape is similar to claws, but their tip is not as pointed.
Teilhardina Brandti, a lemur like early primate that lived about 55 million years ago, was the first primate to exhibit nails on all digits. Its nails allowed the lemur-like animal to grasp onto branches and move through the trees with more agility. The fossils are the first evidence of primates with nails on all digits.5
As civilization progressed and social interactions grew, nails have become objects of adornment and beauty. Good nails are considered to be sign of good health. Nail manicure has been there for more than 4000 years and women take care of their nails more than men.
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Evolution of nailpolish: There is not an exact record of the invention of fingernail polish, but it is believed to have originated in China, somewhere around 3000 BC Early batches were made from mixtures of bees’ wax, gelatin, egg whites and gum Arabic. Members of the Egyptian upper class were also using fingernail polish around this same time; their mixture resembled lacquer paint, and was used to represent money and prosperity. Around 600 BC, the royal colors of China were metallic, gold and silver. These were also the colors of choice for nail polish. The lower class in China was not allowed to wear polish on their fingers, and could be sentenced to death if they were caught. In the 1920s, Michelle Menard was inspired by the invention of new automobile paint, and she used this inspiration to modernize fingernail polish, giving it the look that we are now familiar with.6
References
- Hamrick MW. Development and evolution of the mammalian limb: adaptive diversification of nails, hooves, and claws. Evol Dev. 2001;3(5):355–63.
- Hamrick MW. Functional and adaptive significance of primate pads and claws: evidence from New World anthropoids. Am J Phys Anthropol. 1998;106(2):113–27.
- Soligo C, Müller AE. Nails and claws in primate evolution. J Hum Evol. 1999;36(1):97–114.
- Oldest Evidence of Nails in Modern Primates. Science. Daily; 2011.
- http://www.ehow.com/ The History of Fingernail Polish.