First, three simple realities.

1. Individuals vary in “traits”, which are heritable.

There is enormous variation in the way each of us look, think and act. Some are taller than others; some are quick-tempered whereas others are poised. These distinctive “traits” – physical, and to some extent, mental – are passed on from one generation to the next (tall parents usually have tall kids).

2. Only “fittest” traits are favored.

Resources are always limited. There is never enough food, shelter or clothing available for all of us to enjoy the same high quality of life. Because individuals vary in traits, only those with traits that are good “fit” for the prevailing environmental conditions have advantage over the others.

3. Environment always changes with time.

Everything about our environments – temperature, humidity, growth/destruction of forests – keeps changing in every passing hour, day, month and year, going back to millions of years. Day-to-day, and even year-to-year, changes are small; but, over many thousands of years, little changes add up to create large shifts in the environment.

These three facts together generate the mechanism of Evolution.

4. Shifting environments favor different traits at different times.

A population has a range of traits available at any given time (point #1), some of which are better fit to one type of environment, and others to a different environment. Because environments are changing continuously (#3), traits that are good fit today will become unfit after a long enough time.

5. Descendants evolve from ancestors by inheriting fittest traits over many generations.

Over thousands of generations (assuming one generation to span 30 years on average, this means a time scale of 30,000 years or longer), less fit traits are slowly replaced by fitter traits in the population. This is because unfit individuals die faster, and more good traits are passed on to succeeding generations. After a long enough time, the evolved descendant population will be different from its ancestor.

As an example, consider the height of the present 20+ US population, which ranges from under 5ft to over 7ft, with a mean height of 5ft 7in (averaged over males and females). Now, imagine a world where they spend entire life standing upright in neck-deep water, which is at 4ft now and rising very slowly. This changing environment will favor tall people, who will in turn have tall kids. After many generations, mean height of the descendants would be larger than the current height.

Take-home messages:

The two key things to remember are:

1. Evolution is a slow process. Accumulation of fitter traits, and removal of unfit traits, occur at such a slow rate that it takes thousands of generations to see any difference.
2. Evolution is never perfect. There is never enough time for the population to get rid of unfit traits entirely. Long before this can happen, the environment will shift again, and what is fit now will be unfit tomorrow. So, our population always has a mixture of fit and unfit traits.

Now, a real example. The picture (click to enlarge) shows how we gradually evolved from ape-like hominids, who were our earliest known human ancestors, over a period of 3.5 million years (about 110,000 generations).

As it turns out, this change in our overall look also coincided with a more profound change in our brain size. This is what we look at next.