53: Evolution, the hand and useless body parts

Hello and welcome back to Notebook from the North. This week we will be having a look at how creatures have changed over time to give us the huge diversity of life we see today, as well as what some of the traces of these changes are, and why not all changes are useful. It’s definitely not an exhaustive article, there’s plenty more details and complexities involved in the process of evolving. This is more a brief overview and introduction to this huge topic. Enjoy!

File:Tree of life SVG.svg
A phylogenetic tree of life, showing the relationship between species made in 2006. The centre is the last universal ancestor of all life on earth. The different colours represent the three domains of life- pink is eukaryota (animal/plant/fungi), blue is bacteria and green archaea. (Picture: Iletunic/Wikimedia Commons/Public Domain)

Overview of evolution

Evolution is basically the idea that characteristics get passed down through generations, and certain characteristics might be favoured over others, leading to small, and eventually even quite large, changes in a population over time. One way this happens is through mutations. We are all filled with genes which give our bodies instructions on how we look as well as how we function, and all of us have some kinds of mutations in these genes- mutations being changes in the genes from their original form. This might give us a different hair colour, face shape or determine whether we have freckles or not. It can also have less visible and more practical effects.

How does this affect evolution? Well, for example, you might get a species of small animal living in a forest who doesn’t see very well at night. However, one might have a mutation which makes its eyes better at seeing in the dark. Because of this, it is less likely to get hunted and so lives to a ripe old age, making lots of babies who get the genetic instructions for stronger eyes, whilst other members of its species with weaker eyes are getting hunted and not having as many babies. This is natural selection, or survival of the fittest. The creature with the best adaptation to the environment is the ‘fittest’ so goes on to survive and spread its traits in the population. Over time more members of that population have that trait until many years down the line it is a common feature of that species. Note here that survival of the fittest doesn’t necessarily mean strongest or fastest, but survival of the creature best adapted to it’s environment.

Of course, it was randomly lucky that this trait of sharp night eyes was useful. There will have been many other mutations in the species too e.g. nice new colours or a longer tail, but if these mutations didn’t especially help the species to survive, they wouldn’t get passed on in the same successful way, so either would randomly linger in some members of the population, or just disappear over time.

Within the space of a few generations you are likely to get little change- maybe more little forest animals will have slightly better eyesight. However, if you look at the timescales of life on earth i.e. hundreds of millions of years, there is room for quite a lot of change to happen.

Some mutations are more obvious that others. This cat with different coloured eyes has a very visible mutation, but we all have a bunch of mutations, some of which only affect our appearance whilst others might affect the way our bodies function.

The Hand

Let’s explore how mutations and natural selection can change things over a much longer period. A long, long, long time ago there was a creature roaming the earth which was the common ancestor of many creatures today, such as humans, turtles, bats and whales. As it was a common ancestor which evolved into all these difference creatures, it means they must all have similar features that have just been tweaked and mutated a bit over time. On the outside of course, a whale and a bat look very different, they move in different ways and live different lives, but you can still see their connection in their bone structure, especially the bones of the hand.

The human arm is made up of one long bone at the top, the humerus, then two long bones parallel to each other below the elbow, these being the radius and ulna. Then we have multiple little bones of the wrist (carpals), and finally the bones which make up our five fingers or ‘digits’ (metacarpals and phalanges). Whilst the shapes and sizes have changes slightly, this is actually similar in turtles. They have one long bone, followed by two long bones, followed by some wrist bones and then the bones of the five digits. Sure they have a rough scaly-ish skin covering all this and they use their ‘hand’ in a difference way to us, but it is the same basic structure. It’s the same in bats too, except that four of their five digits have stretched out to become really long compared the other bones, and the skin between these digits has stretched out to become wings. Dolphins again are similar though their one and two long arm bones are a little stumpier, and some of the bones of their digits have fused together. In birds and horses there has been even more fusing, so it looks like they only have one digit and horses only have two single long bones in the arm. All of this makes a lot more sense if you look at the picture below. All have the same basic structure showing that despite all the mutations they have a common source.

Is this just a useful set of bones that happened to be present in all of them because it was practical? Well, not really. If you look at an insect compared to a bird, both fly but they do so totally differently. A bird flies with its arms, whilst an insect flies with a totally different structure that doesn’t look anything like how our bones are setup. Indeed, little of its body looks like ours. Why? Well, humans, bats, birds and horses are all vertebrates- creatures with a backbone. This means we are much more closely related to each other than to insects which are a totally different evolutionary line. We probably had some kind of common ancestor with them a long time ago, but this may have even been before we had bones so mutated bones is not something we share with them. Who knows exactly, it was a long time ago.

Of course, I should mention that the inter-relatedness of these creatures and their evolution is based on a lot more than similar looking hands/hooves/flippers, but this is just an example to illustrate it.

Bones in the forelimbs of a rang of creatures. (Picture: Encyclopaedia Brittanica)

Vestigial traits

So we know what happens when we develop useful traits, which thanks to natural selection and further mutation get taken on to create new shapes and functions. What happens if evolution gives us something, well, a bit useless. This does happen, and we call these somewhat useless traits ‘vestigal traits’. An excellent example is the pelvic bone of the whale. The pelvis is rather useful to us, it holds our legs in place and lets us walk. Whales, evolving from a land creature, also have a pelvic bone as it used to be useful to this ancestor. However, as they swim about now using their tail for propulsion, this pelvic bone is pretty useless. It’s a vestigial trait. What do they do with it then? Well as it serves no function and it takes some energy and resources to make, over time the size of the pelvic bone has dramatically shrunk, and it’s no longer even attached to the rest of the main body like the spine. It just sort of sits there, perhaps slowly being phased out by evolution.

On the flip side, we still have a tail bone, though we really don’t need it. This is because our ancestors likely had a tail which helped them with balance, but we don’t really need that any more so it is much shorter, not even protruding out of our skin, meaning we waste less energy on developing those bones.

Another example is the appendix. Many herbivorous animals like horses and koalas have what is called a cecum, an extra bit of the intestine that helps them to digest pretty nutrient poor foods like leaves and grass. Darwin suggested that the appendix is the shrivelled remains of a cecum, a vestigial trait that we no longer need, as we can see with people surviving after having their appendix removed. However, more recent research has suggested that the appendix is in fact still useful, providing a valuable role in storing bacteria that help to fight pathogens. Whilst we no longer rely on it to help us digest tough leaves and grass then, it is not a totally useless vestigial trait for us.

The whale doesn’t use it’s ‘hind legs’ so these bones and the pelvis have become a little useless, a vestigial trait. They have dramatically shrunk in size compared to the rest of the bones in the body, but haven’t completely disappeared.

Mutations, and the passing on of the most helpful mutations, has led to big changes in the creatures on our planet, allowing new species and subspecies to develop and has given us the huge diversity of life we see today. We are not all the same, indeed many forms of life, us included, are very distinct from each other in many ways too, yet we have a shared history. Biologically life is connected, and sometimes if you look really close you can see some of those connections.

For more info:

There are many different ways to investigate evolutionary relationships. Some examples are given in this video:

Music: kongano.com

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