Small organisms have been around for billions of years, long before the emergence of larger, more complex animals. The first evidence of single-cell organisms dates back to around 3.8 billion years ago, shortly after the Earth had cooled enough for organic life to thrive. In contrast, multicellular animals evolved less than a billion years ago, with larger and more complex animals appearing a little over half a billion years ago. Throughout Earth’s history, small organisms, no larger than the diameter of a human hair, have dominated the planet.
Large animals, on the other hand, tend to take longer to grow and reach maturity, resulting in slower reproduction rates. For example, mice have a short generation time of about 12 weeks, while elephants take closer to 25 years to reach maturity. Additionally, large species evolve more slowly and may struggle to adapt to long-term changes in the environment. They are also more vulnerable to mass extinction events, as seen in the aftermath of the asteroid impact that wiped out the dinosaurs 66 million years ago.
The advantages of being larger include the ability to evade predators, hunt prey, outcompete rivals, and endure temporary hardships. Larger organisms are also better at conserving heat and have a greater potential for intelligence. However, there is an upper limit on cell size due to the mechanics of cell division. Bigger cells have less surface area per unit of volume, making it challenging for molecules to diffuse in and out of the cell without a transport system.
Despite the evolutionary trend towards larger sizes, many groups of animals originate at relatively small sizes. The earliest mammals and some of the earliest dinosaurs were relatively small compared to their later, gigantic cousins. Plotting the size distribution of major animal groups reveals a positive skew, with most species being closer to the smallest size within their group. Insects, with around 5 million species, are arguably the most successful animal group on Earth, with most species being relatively small in size.
While larger organisms have their advantages, the simplest and smallest organisms possess incredible abilities that larger organisms lack. Extremophiles, such as archaea and tardigrades, can survive in extreme environments that would be lethal to larger organisms. Some scientists believe that microbes could survive interplanetary journeys inside meteorites, suggesting that life elsewhere in the Solar System may have a common origin with life on Earth, starting out small.
In conclusion, small organisms have been a dominant force on Earth for billions of years, showcasing incredible resilience and adaptability. While larger organisms may have their advantages, the meek, in this case, the small, truly inherit the Earth and beyond.
