In the post The real differences between aurochs and cattle I go over the organismic differences between the aurochs and cattle in regards to morphology, development, endocrinology, behaviour and genome. Those differences all are interconnected and were caused by the process of domestication. You cannot alter one factor without altering several others at the same time. Each of those factors, like changes in hormonal activity or development, have a particular impact on the organism. With this post, I want to illustrate these impacts by turning a wild aurochs into domestic cattle step by step. I do not say that these changes evolved in this particular order, rather they evolved more or less at the same time but varying extent, this is just a scheme. Please do not use my drawings without permission.
This is the starting point, a wild aurochs:
Changes in thyroid hormones not only caused a more relaxed, lethargic behaviour but also reduced limb and head size, making the overall appearance of the animal more elongated. Probably horn size would be affected as well. Also, the animals do not grow to full size anymore.
Another consequence of hormonal changes, of corticosteroids in particular, would be reduced muscling, a hanging spine and an enlarged intestinum. It becomes apparent that this is the factor that causes the animal to truly look domestic – the deformed skeleton and reduced muscling simply does not look compatible to a wild animal. The drawing above reminds me of a Sayaguesa bull, by the way.
The delay in development causes a phenomenon called paedomorphy, resulting in a calf-like skull morphology, shortened horns and reduced sexual dimorphism. Also the shrinkage of the hump might be caused by developmental delay. A lot of cattle landraces, like Rhodopian short horn for example, look like this drawing.
Genes affecting behaviour also affect other factors, such as coat colour. As a result, the animals will start to show spotted colour because the activity of pigment cells is distorted. Furthermore, horn shape is affected as well, they start to curve more upwards and outwards as in all domestic bovines.
New mutations for new phenotypic traits evolve. Such as for changes in horn shape or curvature, horn size (smaller or larger), stubby legs, shortened snouts, overlong hair, curly hair all over the body, and a whole palette of new colour mutations. The skin also gets more flappier with enlarged or elongated appendages. Cows evolve a dramatically enlarged udder as a result of direct selection on this trait.
Adding all these factors, we get the domestic cattle that we have today.
And here is an animated GIF for this transformation:
For deeper background information, see these posts: