What is small really seems to be beautiful in terms of evolution.
The largest dinosaurs, pterosaurs and mammals may look impressive, but these giants are far outnumbered by microscopic bacteria, algae and single-celled fungi. Small organisms are also ancient and incredibly resilient.
The first traces of single-celled organisms date to around 3.8 billion years ago, shortly after the newly formed Earth cooled enough for organic life to emerge. Multicellular animals evolved less than a billion years ago, and larger, more complex animals appeared just over half a billion years ago.
For most of Earth’s history, the planet was dominated by organisms no larger than the diameter of a single human hair.
Larger animals tend to take longer to grow and reach maturity, so they reproduce more slowly. While mice have a short generation time (the time it takes for a newborn to grow and give birth) of around 12 weeks, elephants take closer to 25 years.
Larger species tend to evolve more slowly and may be less able to cope with long-term changes in the physical and biological environment. Larger organisms also tend to fare worse during mass extinctions.
Nothing bigger than a house cat survived the asteroid impact that wiped out the dinosaurs 66 million years ago.
Being very large requires much more specialization and slower reproduction, reducing the chances of surviving environmental upheavals. For example, larger vertebrates need disproportionately thicker bones and larger muscles. A shrew the size of an elephant would quickly break its legs if it tried to walk.
It is therefore not surprising that many groups of animals appear to have originated at relatively small sizes, and the earliest branching representatives are usually quite small.
Sister groups of winged insects include the tiny springtails (mostly less than 6 mm), while microscopic tardigrades or “water bears” are the sister group of arthropods (which include spiders and crustaceans) and worms. velvet.
The first mammals and some of the first dinosaurs (like Eoraptor measuring less than two meters long) were also relatively small compared to their more recent, often gigantic, cousins.
Why bother getting fat?
Being taller has many advantages. Larger size may make it easier to escape predators (elephants and whales have few enemies other than humans), hunt prey, outcompete rivals, and endure temporary hardship.
Larger organisms also tend to conserve heat better (due to their relatively smaller surface area) and have greater potential for intelligence.
But scientists believe there is an upper limit to cell size. Cell division mechanisms break down into very small and very large sizes. All living beings must also face a universal physical constraint highlighted by Galileo. Larger cells tend to have less surface area per unit volume.
This means that the natural movement (diffusion) of gas, nutrient, and waste molecules into and out of the cell is not enough to maintain function without a transport system. These molecules also have to travel further into larger cells.
Building a larger organism therefore involves two things. First, group a large number of cells so that they can work together. Second, creating different specialized cells for different tasks, including structural support, digesting food, and moving elements such as oxygen and CO₂.
The alternative is to become flat or threadlike (like horsehair worms) or thin and flat (like flatworms). These animals do not need an internal transport system because none of their cells (or their contents) are far from the surrounding air or water.
Paleontologist Edward Cope (1840–1897) proposed that individuals of all lineages tend to increase in size over evolutionary time. While this is true from a statistical standpoint, there are many exceptions, and mass extinction events often bring things back to the smaller end of the spectrum.
Plot the size distribution for almost any major group of animals and you will see a surprisingly positive bias: most species are much closer to the smallest size than to the largest size within their parent group, and it There are relatively few large species.
For example, there are more species of insects (around 5 million) than all other animal groups combined, making them arguably the most successful animal group on Earth.
Most insects are beetles, with an average length of around 6 mm. Giant beetles such as the Hercules (17 cm long) and Elephant (13 cm long) beetles are extremely rare.
Small size allows animals to live in a greater diversity of niches and distribute resources more finely, thus grouping more species and individuals in the same habitat space. Insects are masters of this strategy.
The meek will inherit the Earth – and beyond
Despite the tendency of organisms to evolve to larger sizes, the simplest and smallest organisms still possess many incredible abilities that larger organisms lack.
Many of these tiny “extremophiles” can survive in environments that wipe out most other life forms.
Some archaea (single-celled organisms without a nucleus) can withstand temperatures above 200°C around deep-sea vents, while other species can thrive in waters with high concentrations of salt, acid and alkalinity.
Similarly, tiny tardigrade animals can withstand temperatures between 150°C and -200°C, the vacuum of space, desiccation for decades, and radiation doses 1,000 times greater than those needed to kill a human.
There are even tiny nematode worms capable of living under three kilometers of solid rock.
Some scientists believe that microbes could survive interplanetary travel inside meteorites. Scientists also think that any life we find elsewhere in the solar system could have a common origin with life on Earth – initially small.
Matthew Wills, Professor of Evolutionary Palaeobiology at the Milner Center for Evolution, University of Bath and Tim Rock, PhD Candidate in Biology, University of Bath
This article is republished from The Conversation under a Creative Commons license. Read the original article.