Happy times, everyone! It’s Darwin Day – an international celebration of the birth of the world’s most famous and influential evolutionary biologist, who came into being on February 12, 1809.

The evidence for evolution through natural selection is as beautiful as it is utterly overwhelming. Every part of our own biological architectures – from walking upright to the way our eyes work – is a piece of evidence pointing toward Darwin's elegant theory. Rather conveniently, some of these you can physically see or experience with minimal effort, so let’s have the briefest of looks at our favorites.

1 – The Missing Muscle

If you place your forearm onto a desk with the palm of your hand facing upward, and you make your thumb and pinkie finger touch, one of two things should happen. Either a tendon in your forearm rises up or it doesn’t. If so, then congratulations, that tendon is connected to the palmaris longus, a muscle that’s designed to “help with wrist flexion” – essentially, the bending of your wrist.

Not everyone has this muscle; in fact, a small but significant proportion of the global population doesn’t have this muscle at all on one or both of their arms. Although you can find this in primates that often use their forelimbs to move around, it makes no effect in humans whether it’s there or not, which suggests that in humans, this muscle is vestigial – ancient and functionless.

It’s developed in lemurs and monkeys, but shorter in evolutionary cousins that do far less climbing around. A sublime example of evolution in action, per Vox.

2 – Chin Up

This may come as a complete surprise to you, but as noted by The Smithsonian, we’re the only hominids with (true) chins. Despite being closely related to bonobos, chimpanzees, and the like, we possess a bony lump at the end of our lower jaw, but why?

Various hypotheses abound. One study suggests it evolved in response to the way in which we speak; another indicates it helps support the jaw while we chew. One disagrees and instead concluded that it was sexually selected. Either way, we have chins, and our evolutionary cousins don’t, thanks to a quirk of evolution we’re yet to properly comprehend.

3 – A Hairy Situation

Have you ever wondered why, compared to our mammalian cousins, we’re relatively hairless? So have evolutionary biologists, who have wondered why this camouflage-boosting, heat insulating fur has disappeared in humanity.

The leading suggestion is that losing our hair allowed us to cool down faster, freeing us up to hunt, forage, and run about in the blazing heat – something that clearly went hand-in-hand with our ability to walk upright. This has been contested, and other ideas do exist, but it’s the frontrunner for now.

4 – Coping (Or Not) With Higher Altitudes

Although this is something you experience rather than see, it's still a remarkable example of evolution everyone can relate to.

If you’ve ever tried to climb a sizable mountain, you may have – at rest – noticed you’re still curiously short of breath, perhaps even faint. That’s because at higher altitudes, the density of air is lower, which means there are fewer oxygen molecules per breath.

Most of us will struggle somewhat in those conditions, but certain people around the world don’t. Sherpas, for example, are far more proficient higher up, but why? A recent study found that it’s because their mitochondria – the powerhouses of our cells – use oxygen more efficiently to produce ATP, which transports chemical energy to and fro.

The reason they possess this ability is because they’ve lived at high altitudes on the Tibetan Plateau for 30,000 years, and their genomes have been altered in response to that.