In this tutorial I will try my best to explain how I managed to create this image.

The idea of a child painting in this wall was the first that came across to my mind, this idea looks great at first, but the image can be seen very simple so adding the daddy looking at this child's work looks even greater.

At first I didn't know what type of expressions they should have, I thought about a frightened child and an angry man, but this behavior is too modern for those cavemen. Finally and thinking about evolution I thought that the idea of a child discovering the human painting skills to his father (and to all the world) can be good enough.

I didn't even try to search for the caveman sources because I knew I knew that I would not find them, and I wanted to make the image with a cartoonish look, so I decided to draw them.

The first thing we need is a higher wall with the paintings at the bottom, like they had been done by a child

Resize the painted wall image, then copy some of the not painted parts of the picture and paste on top front, continue the copy and paste process, rotate and flip the pieces to avoid repetitions.
Mask or erase the borders to blend the new layers.
Flatten all the used layers and save and forget the background for now, we'll go with it later

Make the sketch, try to have a clear idea of what you want to get since a refined sketch is important now. Don't continue if you don't have a good sketch of the final result or all the further work can be done in vain.

As I have said the idea of a happy child discovering the new skills aquired due the evolution in front of a surprised father looks great for me. I also wanted to give the child a more evolutionized look compared to the father, so the difference among the skilled and not skilled people was evident. For this reason the father is very big and with the arms too long and the legs too short in comparison to the rest of the body

I wanted to make the guys with a very untidy hair, more like an animal than like human hair, I thought that making child's hair a bit tidier than man's hair could be a good detail to make the child a bit more evolutionated than the man

Open the horse hair texture and taking different parts of the long hair in the neck create the budys' hair. The hair is always hard to create and it's hard dealing with it because you can't transform or warp it too much and keep it natural. For this reason you may use different parts of the texture to make them fit better in the new heads, mask or erase the borders keeping the rough aspect

Create the eyebrows, that's easy, you only need to use one small part of the hair.
In two new layers create two shapes, one for the clothes and one for the skin. There is no need to use the pen tool for these shapes, you can do it with a hard rounded brush at 100% opacity and flow for the skin and a rough brush for the clothes.

Remember that clothes are made of fur, so the border of this shape may have a hairy look too

Open the fur texture, transform and warp it to make it fit better in the new suit, apply it to the clothes layer using it as clipping mask, the easiest way to make clipping mask layers is clicking in the middle of the two layers holding Alt key.

At this point we have to draw and paint a lot of shadows and folds in the skin and in the clothes, using mainly the standard Photoshop spatter brushes to make rough borders.

In a new layer above the skin layer in multiply mode begin painting the roughest shadows and borders using a medium brown tone. Use the smudge tool to blend the new painted areas with the existing ones. Now you can flatten these two layers or continue creating new layers above the exiting ones, always in multiply mode and painting with darker brown tones continue improving the shadows. You can also make the new layers using the skin layer as clipping mask to stay on borders.
To finish the skin we need to add some highlights too, we'll do that in the same way as we have created the shadows but this time in a new layer in overlay mode and painting in white.

In the same way we have painted over the skin we'll go to paint on top of the clothes, creating a new layer in multiply mode over the clothes layer, and over the fur texture if you haven't flattened them before. Begin to paint the folds using a dark brown.
Paint also with a soft rounded brush the shadows to give depth in a new layer in multiply mode and add some highlights in a new layer in overlay mode painting with a white soft rounded brush.

Let's go back to the hair now, select the mans hair and press ctrl+shift+j to cut it to a new layer. Go back to the child's hair layer and desaturate it (ctrl+shift+u). Lower the brightness and the contrast to make it darker and increase the contrast on the daddy's one.
With a black rough brush paint some dark tones to increase contrast, paint also some white hair on the daddy's head.

Now let's go back to the background, duplicate the wall layer (ctrl+j) go to Filter > Other > Highpass... apply it with a 5 pixels radius and set the mode for this layer in overlay.
Duplicate the wall layer again and put the copy over the previously edited layer, set the mode of this layer to soft light.
In new layer in overlay mode paint with the same colors the animals of the wall, this will increase the vivacity and the paintings looks fresh in this way.

In a new layer in multiply mode paint the shadows of the guys using a dark brown rough brush with low opacity, create one layer on top of all in multiply mode too, now paint with a rounded soft brush with very low opacity to create some general shadow and some mood.

Finish with a new layer in overlay mode and paint with a white rounded soft brush to improve a bit the mood

Final result.