The origin of life has been a longstanding question pondered by civilizations throughout history, yielding diverse and often conflicting answers. Let's take a look at some of them.
6,000 years ago, in ancient Mesopotamia—present-day Iraq, the people believed in the Enūma Eliš, the Babylonian creation myth. Tiamat, the chaotic sea goddess, fought against the younger gods. Marduk emerged victorious, splitting Tiamat into two halves to create the heavens and the earth. Following this, Marduk created humanity to serve the gods.
4,700 years ago, in ancient Egypt, the people believed that Thoth, a deity of wisdom, magic, and writing, created animals. Khnum, the ram-headed deity moulded humans out of clay and breathed life into them.
2,600 years ago, in Canaan—modern-day Israel and Palestine—the people believed in the Genesis creation myth. Their God, Yahweh, created the first man, Adam, and placed him in the Garden of Eden. He (not she, because God is a male) then took a rib from Adam, and made Eve, the first woman to provide him with companionship. Animals were created by Yahweh to serve humans. This creation myth is foundational to major religions in the world today such as Judaism, Christianity, and Islam.
1,000 years ago, the Maoris—indigenous people of New Zealand—believed that Tāne Mahuta, god of forests and birds, created the first woman, Hineahuone, from clay. Tāne Mahuta then produced children with Hineahuone, fashioning the whole of humanity as we know it today.
Throughout history, across different civilisations, there have been countless stories like these, believed by thousands and millions to be the answer to the question of how life came to be. In fact, many still believe in the literal truth of them. It's interesting to note that these stories are usually more concerned with humans rather than animals, fungi, plants, and micro-organisms like bacteria and viruses, even though most of Earth is populated by those organisms. But who could've faulted them? Afterall, this is to be expected of a pre-scientific world where nobody even knew about the existence of micro-organisms—Dutch scientist Antonie van Leeuwenhoek was the first to have observed micro-organisms in the 1670s.
This image was created with the assistance of DALL·E 3.
Charles Darwin's Evolution by Natural Selection
Oops I missed out on the 5th story...
In the year 1859, Charles Darwin introduced the theory of evolution by natural selection through his seminal work "On the Origin of Species," challenging traditional creation narratives. Darwin's theory proposed a naturalistic explanation for the diversity of life on Earth, suggesting that species evolve over time through the process of natural selection, where advantageous traits are passed on to successive generations. This theory was conceived by Darwin, based on several compelling scientific evidence that he had gathered after decades of observation of fossils and anatomies of organisms (particularly the Galapagos finches). This groundbreaking scientific theory eventually offered a comprehensive and evidence-based understanding of life's development, encompassing not only humans but also the entire spectrum of living organisms, including animals, fungi, plants, and micro-organisms. Previously, the only explanation for life was creation by a creator. As such, Darwin's work marked a paradigm shift in humanity's understanding of the origins of life, which ultimately influenced numerous fields such as biology, genetics, ecology, palaeontology, medicine, agriculture, ecology, anthropology, and even computer science! (evolutionary algorithms for machine learning)
Hmmm...You must be wondering which one of the 5 stories is the true explanation? I would say that the first 4 explanations are easy enough to be understood by anyone aged 7 and above. The last one, evolution by natural selection, is only slightly more complicated, but still easy to grasp. I promise! So for today, before you decide on which is the true explanation, let me break down to you the what evolution by natural selection means. The evidence for evolution will be covered in another blog post on its own.
This image was created with the assistance of DALL·E 3.
Age of our Earth
Before we approach evolution, it is crucial to understand exactly how old our Earth is, or else evolution would be incomprehensible. The question of how old the Earth is has also been one that many have tried to answer. Many trace the timelines of the various creation myths, such as the one in Genesis, before deriving that the Earth is roughly 10,000 years old. However, scientists have gotten a very different answer when applying the technique of radiometric dating—I will explain the technique in further detail in another blog; meanwhile, you are free to read about it online. Take note that there are also other methods of dating such as dendrochronology (tree rings dating) and ice cores. Briefly, radiometric dating is a technique that involves measuring the decay of radioactive isotopes to determine the age of wooden artefacts, rocks, or fossils. By dating the oldest rocks on Earth, the Moon, and asteroids orbiting Earth, scientists have arrived at the value of 4.54 billion years old, with a margin error of ± 1% (Dalrymple, 2001). At this point, I implore you to pause for a moment. Try to imagine how stupendously old 4.5 billion years is. Try you may, but it's safe to say that both you and I simply can't comprehend the sheer vastness of time which has passed on Earth—a million years would already be inconceivable to us! To drive my point, below is a breakdown of what 4.5 billion years actually means.
4.5 billion years is denoted as 4,500 million years, or 4,500,000,000 years.
Based on the fossil record, modern humans existed in Africa around 300,000 years ago. This means that humans have been around for just 0.0067% of Earth's lifespan. Consider this, imagine if you were to watch a movie titled "The Adventures of Earth", which has a showtime of 4 hours and 10 minutes. You gather your family and friends, with your popcorn and snacks! You all obviously will be very excited for the part where modern humans—like you and me— first appear! So as you sit through this movie for a gruelling 4 hours and 9 minutes, you, along with your family and friends, start to assert that there was an error with the movie, because humans have not appeared despite you being down to the very last minute of the movie! You wait and wait...and in the final second of the movie, in the blink of an eye, all of humanity started and ended in a... single second. That is how much time we, humans, occupy Earth! A single second of screentime out of a 4 hour and 10 minutes movie!
This image was created with the assistance of DALL·E 3.
DNA, Genes, and Mutation
Now let's put the mind-boggling age of the Earth aside and delve into evolution at last. Firstly, almost every single living thing is made up of Deoxyribonucleic acid (DNA)—some viruses are made up of ribonucleic acid (RNA). DNA is a complex molecule that serves as the fundamental genetic material in living organisms. It is a long, double-stranded helical structure located in the cells of nearly all living organisms. DNA is composed of four types of nucleotide bases, cytosine (C), guanine (G), adenine (A) and thymine (T). These bases are arranged in different orders, and in pairs, in order to store genetic information. Small stretches of DNA make up genes which determine the traits of the organism— skeletal and muscular features, size of eyes, skin and eye colour and countless others. Genetic code can literally be read one letter at a time, just like a book! Different genes are made up of different sequence of letters, lasting several thousands of letters for a single gene! For instance, the FOXP2 gene, a critical gene associated with language development in humans, consists of thousands of nucleotide base pairs. But due to space constraints, the following is the sequence for a small part of the gene from a human participant (Maricic et al., 2013):
A T T C A T A A A T T C A T
When two organisms mate, both their genes are shuffled together, analogous to shuffling a deck of cards during reproduction. This process is known as genetic recombination and occurs during the formation of gametes (sperm and egg cells) through a cellular division process called meiosis. However, sometimes during this process, there are occasional 'errors' or alterations in the genetic code. These alterations are known as mutations. So, with mutations, new traits within a population of species would arise. These traits can either be beneficial, detrimental, or neutral. So which genes or traits get selected and passed down to the next generation? That will determine on the selective pressure of the environment. With that, let’s move on to selection. traits within a population of species would arise. These traits can either be beneficial, detrimental, or neutral.
Artificial Selection
Before touching on natural selection, we must cover Artificial Selection, also known as selective breeding. It is the process whereby humans breed a particular species—plant or animal—for specific traits to meet various needs. One common example is the domestication and diversification of dogs. Every breed of dog today descended from the grey wolf —Canis lupus. How did this happen? For around 40,000 years, humans have selectively bred dogs for specific traits and characteristics to meet various needs, such as herding, hunting, guarding, and companionship. For example, if humans wanted to breed a dog for tameness, they would deliberately ONLY choose dogs with the desired trait of tameness, for mating. It is important to note that humans cannot see the genes, they simply select those dogs who are more obedient and tamed. In turn, the genes that code for such behavioural traits gets selected to be passed down to successive generations. After each generation of artificial breeding, the genes that code for tameness will be more prevalent among the population dogs, because only the obedient dogs get to mate and pass down their genes. This process of artificial selection is exactly what led to more than 400 species of dogs, all with different shapes, sizes, and behaviour.
Another example is the domestication of plants in agriculture. Cabbage, Kale, Brussels Sprouts, Broccolli, Cauliflower, and Kohlrabi, look extremely different from each other. But, they all come descended from one species called the Wild Cabbage—Brassica oleracea. Similarly, just like dogs, this is due to humans artificially selecting the traits that they desire, and after thousands of generations (unlike humans, each generation of wild cabbage is around 2 years).
What is a Species?
Now, you might be wondering what exactly a species is. A species is defined as a group of living organisms capable of reproduction with each other and must be able to produce fertile offspring. Two organisms are said to be two different species if they are unable to fulfil those criteria. A lion and tiger can reproduce, but the offspring is a crossbreed, either a liger or a tigon, both of which are infertile, meaning that the offspring of lions and tigers who mate with each other, are unable to reproduce. Because of that, a lion and tiger are considered as two distinct species, because while it qualifies for the first criterion, it fails the second. The process in which one species evolve into two distinct species is called speciation. The examples of artificial selection have shown how the Canis lupus speciated into hundreds of breeds of dogs today, and how the Brassica oleracea speciated into various types of plant species. These speciations occurred because of artificial selection. But there is another type of speciation that is not the result of artificial selection. It is a result of a process that lasted 4 billion years, giving birth to the billions of beautiful lifeforms we currently see today on Earth.
This image is taken from http://www.dogica.com/dogevolution.html
Natural Selection
Now that the concept of artificial selection has been elucidated, it is essential to recognise that natural selection operates on a similar principle, albeit without any conscious selection by any being. In other words, there is no one doing the selecting or breeding. In natural selection, the process is driven by environmental factors and the struggle for survival among organisms. Nature, encompassing a diverse array of influences such as climate, predators, and resource availability, becomes the determining force in selecting traits that contribute to the fitness and adaptability of individuals within a population. Beneficial traits get selected for, and detrimental genes are selected against. Once again, nobody is doing the selecting, it is nature who does it.
However, whether a gene is beneficial or detrimental for a species, it depends on the various environmental factors that I've mentioned. Imagine a population of a rabbit-like species. The population splits into two populations because of a newly formed river between their habitat which isolates both populations. Population A goes on to live in greyish mountains, and a mutation giving it grey fur helps it hide from predators like owls, through camouflage. Now, population B, the same species as A, went on to live in brown rocky areas. In this case, if those from population B undergo the same genetic mutation that codes for grey fur, they would be an easy target for owls. As such, those with grey fur from population B will be eaten by owls and soon, only those with brownish fur will survive and pass on their genes, guiding population B to evolve to have brown fur. Therefore, same change in genes but different result, because depending on the environment, a mutation may be beneficial for one particular environment, and detrimental for another.
Continuing the story, after countless generations and millions of years, remember that the Earth is 4.5 billion years old, population A and population B have only been mating with their own respective groups. This means that there will come a time whereby both populations accumulate too many genetic differences. By then, even if you were to put them side by side, they would look completely different and be unable to mate with each other. This is how a species evolves from 1 original species into 2 separate species. The lion and tiger example I mentioned, is a good example, as they both share a very common ancestor (original species) around 3.9 million years ago (Davis et al., 2010). Right now, they still can mate with each other, but their offsprings, the ligers and tigons, are infertile. As such, I mentioned that because of this, they are two distinct species. Soon, perhaps after a million more years, the lion and tiger would accumulate sufficient differences in their genes, that they would not even be attracted to each other for mating.
Microevolution refers to the small-scale changes that occur within a population over a relatively short period, often on the scale of generations.
Macroevolution encompasses large-scale patterns and processes in evolution, typically occurring over extended periods of geological time. It involves the evolution of new species, genera, families, and the overall diversification of life. The evolution of wolves to dogs is an example of macroevolution. The evolution of whales from 4-legged land mammals, living around 50 million years ago, is another example of macroevolution, but we will touch on the evolution of whales next time.
Note. This diagram displays the four key stages of natural selection that form a cycle, that over time will cause a population to evolve. Here the beetles are all in the same population with a variation of their physical characteristics. Here the brown colour is a more adapted characteristic as it allows the beetle to blend in with its environment and more likely to survive predation. As the brown colour beetle is more likely to survive it is more likely to pass on its genetics to its offspring and overtime as more and more brown beetles survive and the other colour beetles do not the population will shift to being a majority of brown beetles.
Author of Image and Description: Nicholas Toal
Evolution is not Random!
One common misconception is that evolution is by chance and random. This is a grave misunderstanding. Evolution consists of two parts: (1) Mutation; and (2) Selection. it is true that the first part, mutation, is random as the "errors" (beneficial, neutral, or detrimental), occur by chance. However, the second part of the evolutionary process, selection, does not occur at random as it depends on the specific environmental factors in which the species is exposed to. That is the exact opposite of random! It is because of natural selection, that's why all species are adapted to fit in their particular environment—fishes evolved fins to swim faster, monkeys evolved long arms to grab branches. The ocean selects for organisms that can swim faster, and the sun and soil select for the strong sturdy trees. The strong sturdy trees then select for animals that can climb trees. Below is an image showing how horses evolved over the past 55 million years as nature selected for hooves that allow it to run faster to escape from predators.
All Living Things Evolved from a Common Ancestor
By now, you should have a clear undestanding of how evolution by natural selection works. This same process explains the biodiversity that we see on Earth today. The notion that humans evolved from ancient apes is probably familiar to you but, where did these ancient apes come from? The answer is that ancient apes like Australopithecus, which lived around 4 million years ago (MYA), evolved from monkey-like ancestors such as Aegyptopithecus, which lived around 50 MYA. That is why all apes, including us humans, have a tailbone (coccyx). It is a vestigial structure—functionless remnants of ancestral structures. What did the monkey-like ancestor evolved from? The answer is that they evolved from shrew-like creatures such as plesiadapsis which lived around 90 MYA, alongside the great dinosaurs. These shrew-like creatures evolved from intermediary species between reptiles and mammals, like Cynognathus (260 MYA); and they evolved from reptile-like creatures such as Westlothiana (338 MYA); which evolved from amphibian-like creatures such as Ichyostega (360 MYA); which evolved from a fish-like amphibian such as Tiktaalik (375 MYA); which evolved from lungfishes like Coelacanth (420 MYA); which evolved from worm-like creatures such as Pikaia (540 MYA); which took billions of years to evolve from protocells around 4 billion years ago. The Last Universal Common Ancestor (LUCA) is the most recent common ancestor of all life on Earth, and it lived more than 4 billion years ago. It represents a point in the evolutionary history where all living organisms, including humans, plants, fungi, and bacteria, share a common lineage.
Reversing this evolutionary sequence, we trace a 4-billion-year trajectory from LUCA, to the complex diversity of life forms observed today, including humans. Beginning with LUCA, life evolved into uni-cellular organisms, progressing to multi-cellular entities and subsequently to worm-like creatures. These entities gradually transformed into fish-like organisms, developing lungs to become lungfishes. They then moved out of the ocean to conquer the land, acquiring webbed fingers to transition into amphibian-like beings. The evolutionary path continues with reptile-like creatures, characterized by dry skin, distinct fingers, and amniotic eggs, leading to shrew-like mammals with hair and live-birth. Further adaptations including the evolution of bigger brains and bipedalism, accompanied by the loss of the tail structure, ultimately led to the evolution of ape-like beings such as humans.
See...told you that I needed to explain the age of the Earth before I moved on to evolution, if not you would think that millions and billions were typo errors in this blog.
Below is an inforgraphic that shows the one out of billions of lineages that led to humans:
Source of Image: Visual Capitalist
Below is an image showing the phylogenetic tree of life—not drawn to scale obviously because there are around 8.7 million species alive today and more than 99% of all species to have ever lived have gone extinct. Not that this particular image only depict animals. Plants, bacteria, and fungi split off before the single starting point that you see on the left.
Source of Image: Leonard Eisenberg
Conclusion
To sum up, Charles Darwin's evolution by natural selection is the process in which how all life forms, including humans, came to be how they are today, through the non-random selection of genetic traits that arise from random mutations. In this blog, I have explained what are DNA, genes, and mutations. I have also touched on artificial selection, using dogs and Broccolis as examples, before moving on to natural selection. I then end by showing how all life forms are related to each other—dogs and broccolis are our ancient cousins, with the latter sharing a more ancient common ancestor with humans and dogs, around 1.6 billion years ago, when plants, animals, and fungi diverged from a uni-cellular ancestor (Wang et al., 1999).
If you still find it unbelievable that a worm could evolve to become a human, consider this, it took only 40,000 years for a carnivorous Grey Wolf to evolve into a harmless Yorkshire Terrier. Both of their appearance and behaviour are drastically different from each other. The Grey Wolf would be able to devour the Terrier, and its human defender, in an instant. If that kind of evolution can happen in 40,000 years, which is in fraction terms, assuming Pikaia's age of 540 million years as the denominator, a negligible 1 out of 13,500, it would then not be far fetch at all to imagine how a worm-like creature could evolve into human beings, given the sheer vastness of geological time available for evolution to occur. Unfortunately, the evidence for evolution is far too immense to cram in this blogpost and as such, they WILL be covered in a future blogpost.
To me, the interconnectedness of all living beings highlights humanity's obligation to treat other species with compassion, preserve the Earth's biodiversity, and transcend the self-centered perspective inherent in our human-centric worldview. Furthermore, the beauty of this remarkable fact, that is evolution, is that you are a product of an unbroken line from the first cells living 4 billion years ago. At every step, every single one of them reproduced, despite facing the harsh environment of Earth, running and hiding from predators, and overcoming violent natural disasters. It is because every single one of them reproduced, it has led to your birth and existence in this world. I find this to be deeply poetic and in fact, I think evolution is much more beautiful than any creation myth that was ever told. Another contributing factor for its beauty, is that it is based on evidence and true. I'll end off this blogpost with Charles Darwin's famous ending paragraph in his book "The Origin of Species."
“There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” ― Charles Darwin, The Origin of Species
References
Dalrymple, G. B. (2001). The age of the Earth in the twentieth century: A problem (Mostly) solved. Geological Society, London, Special Publications, 190(1), 205–221. https://doi.org/10.1144/GSL.SP.2001.190.01.14
Davis, B. W., Li, G., & Murphy, W. J. (2010). Supermatrix and species tree methods resolve phylogenetic relationships within the big cats, Panthera (Carnivora: Felidae). Molecular Phylogenetics and Evolution, 56(1), 64–76. https://doi.org/10.1016/j.ympev.2010.01.036
Maricic, T., Günther, V., Georgiev, O., Gehre, S., Ćurlin, M., Schreiweis, C., Naumann, R., Burbano, H. A., Meyer, M., Lalueza-Fox, C., De La Rasilla, M., Rosas, A., Gajović, S., Kelso, J., Enard, W., Schaffner, W., & Pääbo, S. (2013). A recent evolutionary change affects a regulatory element in the human foxp2 gene. Molecular Biology and Evolution, 30(4), 844–852. https://doi.org/10.1093/molbev/mss271
Wang, D. Y.-C., Kumar, S., & Hedges, S. B. (1999). Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi. Proceedings of the Royal Society of London. Series B: Biological Sciences, 266(1415), 163–171. https://doi.org/10.1098/rspb.1999.0617
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