The skin of your feet
In prehistoric times, early man walked everywhere he went, giving rise to thickly calloused, hairy and durable feet. As he walked from one edge of the Earth to the other, the thick pads on the bottoms of his feet would have absorbed more punishment than most modern day members of the human race can imagine. Trekking through vast wilderness and endless primitive jungle, early man was likely as far removed from today's modern man, born of privilege and ease of living in today's world of technology and abundance of wealth, as can possibly be conceived.
As time marched on, coverings were invented for the feet, protecting the soft skin from briars and thorns as well as temperature extremes, while doing little to keep the feet themselves clean or free of grime and dirt. These primitive shoes were sufficient to prevent serious injury to the feet and that was likely the extent of their role at first. But as more time passed and more advances were made in the art of crafting shoes, the highly practical application of the shoe in an effort to protect the foot became overwhelmingly popular. Shoes and boots protected the feet and the skin from damage, and shoe makers and cobblers produced more and more advanced footwear that resulted in all manner of protection from the elements, cold, heat and dirt, as well as still fulfilling the original intended use of protection from harm.
In the modern day, due to the advance of shoes and their ability to protect the user's feet from damage in a comfortable, padded shoe, the average set of feet have long since evolved into soft and supple things and are nothing like the feet of our primitive forebears. The skin is not nearly so thick or tough, and requires great care to preserve the skin and nails into old age. While few consider the concept to be critical, caring for the skin of your feet is very important. The feet can be afflicted with all manner of disorders and problems, such as athlete's foot and jungle rot, as well as a vast array of more mundane and domestic afflictions. Caring for the feet and the skin on the bottom and top of the sole can mean the difference between retaining their use into old age, versus encountering difficulties walking when entering into the golden years.
Fortunately, the treatment and care of your feet is dead simple and requires no special equipment, tools or training. Soaking the feet in hot water with epsom salts is a good first step and should be done as often as feasible. Once the feet have dried, simply apply a rich emollient or lotion to the entire foot, especially the bottom of the pads, where it is most likely to be in the worst condition. Continue this process, reapplying the lotion or cream as frequently as is realistically possible and always taking the extra step of scrubbing them at length when bathing.
Proper care of the skin should always include the feet and most lotions and ointments can safely be used on the feet without fear of harm.