Slow worm (a legless lizard)
People tend to equate change with progress. Since evolution is the process of change over time, such thinking sees it as a progressive process that leads, for example, to increased complexity. Common examples of evolutionary change include: the emergence of multicellular organisms from unicellular ones, increase in heart complexity during vertebrate evolution (from fish heart with two chambers to mammalian and bird heart with four chambers), or the increase in brain volume and cognitive abilities in human evolution.
It is undeniable that in the beginning life on earth was simple and that more complex (e.g. multicellular) organisms evolved from simpler (unicellular) ones. However, we should not forget that in the process of evolution simpler forms can also arise from more complex ones.
Some research even indicates that reductions in complexity of form have occurred over the millennia just as often as increases in complexity. After all, it is not the more complex organisms that survive, but the ones that are better adapted to the conditions in which they live*.
To balance out the human tendency to focus on increasing complexity a bit, here are some examples of evolution leading to simplification.
Disappearing limbs
The fins of cetaceans (whales and dolphins) are simplified versions of the limbs of the land mammals from which they evolved (did you know that their closest living relatives are hippopotami?). In the process of evolution, the forelimbs have been reduced: they have fewer finger bones and the fingers themselves are fused together. Additionally, the hind legs have completely disappeared, and what remains are only the remnants of pelvic bones, deep under the skin.
The situation is similar in other aquatic mammals, such as manatees and dugongs.
Some terrestrial animals have also lost their limbs, for example snakes and legless lizards.
Life of a parasite
Life inside another organism is often associated with organ loss. For example, in the process of evolution tapeworms lost their digestive tract, brain and probably many sensory organs, which are unnecessary for life inside the intestines of other animals.
There are hundreds of thousands of species of wasps in the world that are parasitoids of other insects – they lay their eggs in the eggs or larvae of other insects which are then consumed by the wasp’s offspring. Many of them are very small (below 1mm) and can even lack a heart. This group includes also the insect with the smallest known nervous system – the Megaphragma mymaripenne wasp (see picture above). It has just over 7 thousand neurons (contrast this with, for example, the nervous system of a honeybee which consists of around 850 thousand neurons). Additionally, 95% of adult neurons do not have cell nuclei – a feature unique in the animal world. Despite this, adults are able to fly, and find food and the eggs of other insects in which they will lay their eggs.
Sedentary lifestyle
A sedentary lifestyle seems to require less mental capacity than an active life. And while neither currently-living corals nor their ancestors ever had a brain, the case is different for bivalves (group including clams and mussels).
Bivalves have no central nervous system or head, although their ancestor most likely had one, as do now their close relatives: the cephalopods (like octopus and cuttlefish), which are among the most intelligent animals we know. Interestingly, there are now many more species of bivalve molluscs than cephalopods, showing that the headless life can be very effective in many environments.
The success of simplicity
The fact that greater complexity is not necessary for evolutionary success can also be seen in the fact that many currently living animals are similar (at least in structure) to their ancestors. For hundreds of millions of years, jellyfish have lived in ocean waters, and nematodes in the soil and other organisms. And they are not some individual relics of the past, but groups of thousands of species living all over the world.
And although it sometimes seems that humans (‘the pinnacle of evolution’) rule the world, there are many more bacteria on it, without which we would not survive, and who are extremely likely to carry on with no trouble at all when we become extinct.
* The increase in complexity can even have negative effects. Our big brain and head can lead to complication in childbirth and even death of the mother or the baby.
The post was inspired by an article: Endless forms most stupid, icky, and small: The preponderance of noncharismatic invertebrates as integral to a biologically sound view of life.
Photos:
Padalec: By Holger Krisp – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=27245232
Dolphins: joakant, Pixabay
Wasp: By Alexey A. Polilov – Polilov AA (2017) Anatomy of adult Megaphragma (Hymenoptera: Trichogrammatidae), one of the smallest insects, and new insight into insect miniaturization. PLoS ONE 12(5): e0175566. doi:10.1371/journal.pone.0175566, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=69934221
Mussels: stux via Pixabay
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Brilliant: sedentary life leads to brainlessness 🙂
Evolution seems to take longer, but I sometimes seem to notice this happening in humans.
🙂 Indeed, unused organs atrophy. Let’s keep our brains active!
How interesting and exciting…thank you.
Thank you. I’m glad you liked it.