Our distant ancestors had, as you can imagine, a very different diet than we do now. Not only were there "no phones, no lights, no motor cars"- there weren't any processed foods in shiny plastic bags, or soft items like marshmallows, and many ancient peoples ate a lot more raw food than they did cooked. (Most cultures hadn't discovered coffee yet either, which is probably why it took so many thousands of years to invent the iPad.)
Ancient peoples used stone implements to grind their grains. The stone grit and dust got into their food and that, combined with the greater crunch in the average meal to begin with, led to very rapid tooth wear. Basically, most ancients had flat back teeth by the time they were in their twenties. Here's a photo I took while visiting my friend Dr. Janet Monge at the Museum of Archaeology and Anthropology at Penn:
Contrast that with the very model of a modern human molar- look at the second molar in this photo, the one without a restoration, all the way to the left:
So- human beings were "designed" to end up with flat back teeth at a very early age, and nowadays we usually keep our hill-and-valley cusps and grooves for most of our lives.
What happens with restorations on teeth? Well, porcelain crowns wear down at a slower rate than natural teeth. There is always concern that they will excessively wear the opposing tooth, though I rarely see any significant problems from this in practice. (After all, they're supposed to be flat, right? But it rarely amounts to anything damaging.) Composite resin restorations wear down a little bit faster than natural teeth, though problems of rapid wear of these materials were solved back in the late 80's. Here's a 5-year-old pair of composites and an older porcelain crown; as I watch these over the years in practice, they don't change much on a practical level:
Silver amalgam restorations wear down faster than a modern composite resin. They also often don't get replaced as soon as they should because they mask problems that lurk underneath. So we end up with this:
This old amalgam is deep! It has worn down deep in the center, and what's worse, the upper tooth has drifted down into this (because it can) and the cusps of these two molars are now much steeper than they should be. Remember- the natural process is age=flatter. Here we have age=steeper.
Lots of times, the high lateral pressures that result from this effect result in fracturing away of a cusp or two, making an expensive crown necessary. Sometimes the molar even cracks vertically right down the middle and has to be extracted. Bite problems often result as well, as the cusps interfere with each other during chewing and cause muscle pain.
I was taught by some very bright professors to correct these effects whenever I have the chance in the course of doing necessary dentistry. So:
-As I restored this tooth, which had a cavity under the old silver amalgam restoration, I re-established proper anatomy of the cusps, particularly the one back and to the left. (Disto-lingual, for the cognoscenti). I also slightly reduced the cusp, not seen here, on the upper tooth that opposed this one being treated. Never making them as flat as paleolithic teeth, of course- just, well, reasonable. Balanced. This careful attention to the details of occlusion, the manner in which teeth come together and function, reduces fractures and muscle pain and leads to a lot less maintenance over the years. Thoughts of this are in my mind each and every time I treat a tooth. A healthy occlusion is an important part of having a healthy mouth.
Finally, having our cusps is splendid, but the repetitive stress of chewing has to wear something, and it's often the outsides of our teeth. The theory goes that the thin enamel at the gumline is weakened by all the sugary things we eat and drink. The sugar, metabolized by germs in the plaque, leads to acidity in our mouths for periods of time that last about half an hour after we finish that chocolate bar (yum) or gummy candy (why waste the calories?) or Gatorade (why? why?).
This weakened enamel apparently flakes off in little tiny bits, but they keep flaking and eventually we end up with a notch, sometimes a very big one:
These notches are called abfractions. Once, I went to visit Dr. Monge and we looked at hundreds of archaeological specimens and I never once saw an abfraction. Not one. Apparently, ancient peoples used to wear the heck out of the tops of their teeth, but without access to Twinkies, the sides remained pristine. The combination of less acid and less lateral stress from those pretty cusps, which were all gone by the age of like 25, meant that abfractions simply didn't occur.
Teeth are so weird, when you get right down to it...