Why do humans grow taller than their ancestors?

Question

I'm taking the Introduction to Genetics And Evolution course and in the first lecture it was said that:

Evolution in a biological sense is simply a change through time. And very importantly, that is change through time over generations. So for example, the human species, hundreds or thousands of years ago was much shorter than it is today. Over the course of those generations, the human species has grown taller. That change is an evolutionary change.

I'm wondering why we become taller than our ancestors? Now we almost don't fight with other species and can live doing something that doesn't require physical strength. It's more logical to me if we would become shorter.

Answer 1

A likely misunderstanding of yours

Now we almost don't fight with other species

Misunderstanding about selection

As you will go through this course, you will understand why this sentence makes little sense. A change in allele frequency via natural selection is caused by a fitness differential among genotypes within a population. The existence of a competitor species would eventually affect this fitness differential but there is no need for a competitor for having a selection pressure.

I think you are having the view that selection is a process that allows a species to be better at coping with a competitor but this is wrong. Selection will increase the frequency of individuals having high fitness in the population. The resulting population may well not be a better competitor.

False opinion that we are not competing with other species

Also, about nearly 50,000 people are dying every day from infectious disease (UN press release). So, we are definitely still "fighting" with other species. Adaptation to malaria in humans is a famous case study (see this answer and its links).

Also, according to reuters (who fail to link to their source), the worldwide cost of pests in agriculture is estimated to about 540 billions USD per year! So yep... competition there is.

What selection pressures are there on human height?

Review paper recommendation

Stulp and Barrett 2016 is a review paper on the evolution of height in humans that you might want to have a look to have a clearer picture than what I will give here. It is easy to read. I highly recommend having a look at it.

They cite a lot of papers. I won't cite them again below but I will just attempt to make a vague summary of the parts which I think will interest you the most.

What selection pressures are there on human height?

The question appears to not be that easy to answer. In humans, height is highly heritable and there is a lot of variance in height among populations. It is likely that different selection pressures may have acted on different populations.

There are a diversity of selection patterns

In short, selection pressures on height include intra and intersexual selection. Taller adults seem to have a reduced mortality rate. Taller women have a lower risk of complications when giving birth. However, in both men and women, the correlation between height and number of offspring varies a lot from study to study (and likely from population to population). In the USA for example, men having the highest number of offspring are of intermediate height (balancing selection).

It appears that epigenetic variation also have an important impact on height Simeone and Alberti 2014 and there might have a sex-driven intra-locus conflict on height (Gilks et al. 2014; Stulp and Barrett 2016)

There is evidence of current selection on height

We have a fair number of evidence that height was and is still under selection in humans but we think that the exact pattern of selection pressures acting on it is quite complex. Note that there is evidence that height is still under selection today.

Answer 2

In my opinion there is a fallacy in your understanding of evolution. First of all evolution is blind to the future. Also living organisms don't undergo evolutionary changes toward something that might appear more logical.

Imagine there were only few tousands exemplars of Homo species left and the planet Earth would change so much that 90% of its surface is under water. It would be practical to have webbed hands. An exemplar with webbed hands could swimm better, and consequently be better equipped to provide food. So what's going to happen. Are the humans going to develop webbed hands? Certainly not. It's not how evolution works. Just because something appears to be logical or of use, it's not going to come overnight. But now imagine because of a certain mutation some exemplars have webbed hands. They are more likely to survive in an environment with large water surfaces. Therefore they are more likely to leave offspring. Exemplars without this mutation wouldn't be able to gather so much food, to escape from predators, etc. Eventually after millions of years they might all die out and only exemplars with webbed hands are existent. Then all humans would have webbed hands. The genes responsible for normal hands as we have, are almost not existent in the population.

After further millions years the water masses have retreated and there is much more land surface. The evolution was blind for this. The humans continue to have webbed hands, although they aren't so practical anymore. Eventually a mutation occurs again, or some exemplars with recessive genes for normal hands have offspring. This offspring now has better chances to survive and in the course of many generations there is a substantial part of the population with normal hands.

The humans are today much taller than earlier, because it must have been of evolutionary advantage for very long time. In order to get shorter as a species, the environment (and also other factors) should favour this.

We have food in abundance, good medical care, and most of the offspring can survive to adult age. This enables most of the people can unfold their full genetic potential and grow taller. The humans (at least in the western world) consume milk after the weaning. It contains growth hormons and stimulates the growth.

In order to get shorter in average as species, the shorter exemplars should be able to live longer and leave considerably more offspring, and there should be evolutionary drawbacks for taller exemplars to leave enough offspring.

Do we have those conditions? No. That is why we don't get shorter.