Primate grooming and flat nails go together.
They probably both evolved at the same time, out of a common will and need.
It was the desire to cure itches, aches and pains, without ripping the flesh with claws, that caused the appearance of flat nails.
But, in my opinion, today's primates are not grooming in a way that could have caused flat nails to evolve.
The first primates were grooming properly, but their skills and techniques have been replaced by a useless social ritual.

We've all seen pictures, films and videos of primates grooming each other.
One individual performs the grooming work (the groomer), while the other (the groomee) takes a stiff pose.
In almost all cases, the groomers are using their sense of sight to guide their actions.

Grooming is the process of unfolding the skin, and cannot be confused with pest and debris removal.
It's sad for me to say that none of the primates above are doing any grooming.
This affirmation is based on my own experience of grooming, since 2002, and the actions it requires.
The two main problems are;
• Their use of sight,
• They aren't doing it individually.
That being said, some primates probably groom themselves properly, as explained below.

Several tasks are performed;
• Removing parasites, most are promptly eaten,
• Cleaning the fur,
• Scraping away skin imperfections,
• ...
The groomer will pick off anything abnormal from the fur of the groomee; lice, ticks, fleas, insects, bits of wood or grass, ...
If something strange is spotted on the skin itself though, it will be investigated with the nail; so scabs, pimples, spots, ... will all be scrubbed off.
This last point is important, because while they work on the skin itself, a bit of real grooming is performed.

The first primates, who developed grooming about 65 million years ago, were small, possibly nocturnal, squirrel-like animals.
They were grooming properly, because they relied only on their sense of touch to perform this activity.

However, during their long evolution, primates started using vision rather than their other senses.
This has resulted in an over-reliance on that sense.

This tendency was detrimental to their grooming technique because; grooming is performed blind.
Looking at the skin is useless and counter-productive, because:
• Folds can hardly be seen,
• If you use your sense of sight, it overwhelms your weak sense of touch.

All grooming procedures can be performed at night, with your eyes closed, or looking elsewhere.
You can feel the folds with your nails, but you can't see them.
Grooming depends on sensations coming from the skin to guide and direct your actions.
As soon as you use your sense of sight, it's so powerful that you rely on it immediately, and your entire technique changes.
This situation is further complicated in other primates by the presence of thick fur.

Grooming lets you use your nails to unfold the skin, and remove dead cells on your body and face.
Here are a few pressure grooming and superficial grooming examples:

The following video will give you a good start:

Human grooming rediscovered - Video

You can often see for yourself whether primates are well or badly groomed.
You just have to look at their skin.
In most cases, primates have visible folds that they wouldn't have if they had, even lightly, groomed the area.

As you can see, you're already an expert on primate folds because you observe them on humans.
However, the folds often aren't visible; they appear as ugliness for years before you can spot them.
Great apes are among the most badly folded of all primates.
Their noses become so flattened that their breathing is surely restricted.

Researchers who closely monitor their activities tell us that they spend, on average, between one and two hours a day grooming.
All species, sub-species, and even some communities have their own habits and refinements.
We can distinguish three types of grooming:
• Motherly grooming,
• Social grooming,
• Self-grooming.
It's the social aspect that leads primates away from grooming properly.

As soon as the baby is born, it is welcomed into this world by its mother's warm tongue, licking its face.
She will remove what remains of the birth membranes, and lick the newborn clean.
A few minutes later, she will start grooming the baby with her fingers and nails, while simultaneously inspecting it.
Several of these grooming sessions will take place daily, and this care is extended throughout childhood, sometimes into adulthood.

Unfortunately, the mother's grooming technique is flawed, since she uses her eyesight, and this prevents her from doing an acceptable job at unfolding the skin.

She may, on the other hand, be doing excellent work at removing pests and keeping the fur clean.

The grooming skills acquired by children should transform into self-grooming habits when they become adults.
But, they don't.
Instead, individuals try to replace their mother's grooming with someone else's, and this becomes social grooming.
Grooming is the most important social activity in almost all primate species, and it plays a major role in diffusing tensions within groups.

Self-grooming is done by one individual in solo.
It is of greater interest to us because it often represents real grooming; the kind that unfolds the skin.
The problem is that self-grooming is very intimate, and mostly performed when one is alone.
Because of this, we don't really know how much self-grooming is actually done.

The emergence of the social aspect of grooming is very similar to what bonobos did with sex.
In bonobo groups, sex has been diverted from its reproductive role, then subjected to social customs.
Similarly, the procedures and rules implemented in social grooming steer it away from its skin-unfolding function.
Social grooming wouldn't be bad if it was performed equally by everyone, on anyone, but it is often based on the rank of the participants.

Nevertheless, social grooming promotes closeness, and «taking care of others» attitudes that certainly have several advantages.
It serves as a cement, holding communities together.
Regrettably, their use of sight totally overpowers the sense of touch that they should be concentrating on.
As a result, very little real grooming is performed, and their skin remains folded.

Grooming became an instinctive behavior in primates at least 65 million years ago.
What has happened to it?
How has it evolved in humans?
The answer is simple; you feel the instinct to groom several times a day; you call it itch, ache, or pain.

But, you react totally incorrectly; by scratching, or ignoring it.
This attitude stems from an over-reliance on sight.
Instead, you should be immediately exploring your skin with your sense of touch.

When the first primates, about 65 million years ago, developed flat nails, this evolution was surely caused by the behavior of their own ancestors.
If you lose your claws, it's probably because you don't use them very much, and a new activity is creating the need to improve this tool.
The sharpness of the claws was removed so they could touch and manipulate their own skin without hurting themselves.
Flat nails also helped them do a better job of cleaning their fur.
So our ancestors, at least those early ones, were possibly peace-loving and non-aggressive, but also clean individuals.

Flat nails represent only one part of the physical changes needed to make grooming possible.
The whole fingertip became a probe with;
• The development of an ultra-sensitive fingertip pad underneath,
• The creation of a «nail bed» under the nail plate.
Packed with nerve endings, it amplifies the sensations coming from the nail.
Grooming also requires certain neural mechanisms to be in place.
Most grooming is triggered by itches and pains generated by the skin of the locations that need it.
You respond to signals.
Also, while grooming, your actions are guided by specialized sensations coming from the skin itself.

Some believe that the first primates, along with their ancestors, were nocturnal animals.
I agree with this hypothesis, because grooming is entirely based on the sense of touch.
It is the sense of touch repairing itself.
So, the individuals doing it must concentrate on this sense above all others.
The sensations that one experiences are personal, and cannot be felt by someone else.
Therefore, I think that grooming developed as a solitary activity.

Just like hooves, fingernails have evolved from the claws of reptiles.
Around sixty-five million years ago, a reduction in the thickness of the deep layer of the claws led to the development of nails in the earliest primates.
Primates are not the only ones to possess flat nails; some marsupials do as well.
This is a good example of parallel evolution, where we observe the development of similarities in distinct lineages.
This demonstrates that flat nails are an important evolutionary goal.

There is an intermediate evolutionary step between claws and nails; it is called the toilet-claw.
The first primates had them on the second digit of their hands and feet.
Just like nails, toilet claws are made of keratin.
Their shape is similar to claws, but their tip isn't as pointed.
Prosimian primates still have toilet-claws today, while the nails of some South American monkey species have regressed to claws.

We've all observed dogs scratching themselves.
Some animals will rub their hide against a rock or a tree, while others will roll over on their back and grind their skin onto the ground.
Of course, parasites could be causing their discomfort, but my observations tell me that these animals experience the same unpleasing itches and pains as we do.
Animals with hooves are very ill-equipped, while those with claws can't press on their skin without piercing it.

Yet, the only way to get rid of these unpleasing sensations is to apply pressure on the skin.
The flattening of the claws had to evolve from a continuous will to act, to work on the source of the nagging feels, without harming the skin in the process.
I do not believe that the primate grooming activities illustrated above on this page are those that led to the evolution of flat nails in early primates, nor that they would be sufficient to cause such a change.