Increased human height, in its most profound sense, represents a genetic variation upon which natural selection may act. If it is advantageous to survival and reproduction, then over time, height will increase. The opposite, of course, is also possible.
As to whether it is advantageous or not, this depends. There are performance trade-offs for shorter or taller stature. These will be explained in the next chapter. To summarize though, taller stature is favorable for reach, visibility, speed, absolute strength, heat retention, and energy efficiency. On the other hand, shorter stature is favorable for maneuverability, agility, relative strength, heat dissipation, and lesser energy requirements.
Depending on our ancestors’ environment then, these trade-offs influenced natural selection to increase or decrease height. In the long run, it is clear that height has trended upwards. We are not alone in this trend towards increasing size. What is referred to as Cope’s rule postulates that population lineages, across all species, tend to increase in body size over evolutionary time1.
Yet this increasing in size occurs most gradually, in small increments. Giraffes, for example, have taken millions of years to reach their current height. This has afforded them, via natural selection, ample opportunity to adapt for this height. This is in stark contrast to tall humans who have most likely reached their additional size above our species’ average in a mere few generations. There are many instances where this differing extent of adaptation between giraffes and tall humans can be observed.
Consider blood pressure. The giraffe, with its powerful heart, has a blood pressure twice that of humans. This makes it possible to pump blood all the way up to its lofty brain. But this blood pressure should drop significantly when the giraffe senses a lion and quickly raises its head from a drinking position. This would cause it to faint and become easy prey for the lion. However, giraffes have adapted via valves in their jugular veins to prevent blood from flowing back down to the head when lowered2, and then other mechanisms to assist in bringing blood up to the head when raised3.
Like giraffes, tall humans also have elevated blood pressures, though only slightly. One study found a 3/2 mmHg increase in systolic/diastolic blood pressure per 6″ increase in height4. Unlike giraffes though, tall people lack unique adaptations for preventing greater pressure differentials. This is why tall people are more likely to experience head rushes and even fainting when going from a horizontal to a standing position (orthostatic or postural hypotension)5,6.
Many other aspects of tall stature can be explained similarly. For instance, running speed increases with height but eventually starts to decrease, despite the general trend in nature where bigger is faster. More consequential though, health issues such as musculoskeletal injuries and cancer both increase and reproductive success decreases. There is an optimal height for health and longevity, and both before and beyond this, outcomes worsen, forming an inverted u-shaped curve. These will be discussed in the chapters to come.
A useful analogy here is the design of buildings. Over time, architects have gradually learned how to design taller buildings as they gained knowledge from past experiences. If an 18th century architect, on the other hand, had just up and decided to out of the blue build the Burj Khalifa, which at least for now is the world’s tallest building, it surely would have been a disaster. Of course this is the extreme case, but much like for tall humans, it holds that unusually tall buildings miss out on the same extent of optimization as buildings of more common heights.
What about height in the long term? As noted above, the general trend in nature is gradually upwards. Yet the laws of scaling do eventually begin to apply downward pressure, as explained in the next chapter. Beyond even this though, mass extinctions tend to be less kind to the largest species, the dinosaurs being the classic example. This is known as the Lilliput Effect. Environmental changes that could cause mass extinctions include famines, epidemics, natural disasters, asteroid collisions, and climate change perhaps from global warming or cooling.
Some have actually suggested that we should intentionally shrink the human race to reduce our environmental toll and decrease the risk of famines. But there are counter arguments to this made in the final chapter, one being that there are other more crucial cultural and technological advancement to thwart such disasters. In the event these are sufficient, perhaps we will in fact continue to grow taller, as per Cope’s rule.
Hone DWE, Benton MJ. The evolution of large size: how does Cope’s Rule work? Trends Ecol Evol. 2005;20(1).
Mitchell G, van Sittert SJ, Skinner JD. The structure and function of giraffe jugular vein valves. South African J Wildl Res. 2009;39(2).
G. Mitchella JDS. How Giraffe Adapt to Their Extraordinary Shape. Trans R Soc South Africa. 1993;48(2).
Voors A, Harsha D, Webber L, Berenson G. Relation of blood pressure to stature in healthy young adults. Epidemiol Am J.
Stewart JM, Clarke D. “Hes dizzy when he stands up”: An introduction to initial orthostatic hypotension. J Pediatr. 2011;158(3).
Arvedsen SK, Damgaard M, Norsk P. Body height and blood pressure regulation in humans during anti-orthostatic tilting. Am J Physiol Regul Integr Comp Physiol. 2012;302(8).