by Small seems beautiful in evolutionary terms. The largest dinosaurs, pterosaurs and mammals may look impressive but these giants are outnumbered by microscopic bacteria and single-celled algae and fungi. Small organisms are also ancient and extremely resilient.
The first evidence of single-celled organisms comes from about 3.8 billion years ago, soon after the newly formed Earth cooled enough for organic life to emerge. Multicellular animals emerged less than a billion years ago, and larger and more complex animals appeared just over half a billion years ago. For most of Earth’s history, the planet was ruled by organisms no larger than a single human hair in diameter.
Larger animals tend to take longer to grow and mature, so they reproduce more slowly. While mice have a short generation time (how long it takes for newborns to grow up and give birth) of about 12 weeks, elephants take closer to 25 years.
Large species tend to evolve more slowly and may be less able to cope with longer-term changes in the physical and biological environment. Larger organisms tend to fare worse in mass extinction events. Nothing much more than a domestic cat that survived the asteroid impact that wiped out the dinosaurs 66 million years ago.
Being very large requires much more specialization and slower reproduction, both of which reduce the chance of secure environmental disturbances. For example, larger vertebrates disproportionately require thicker bones and larger muscles. A dazzle the size of an elephant would quickly break his legs if he tried to walk.
So it is not surprising that many animal groups appear to have originated relatively small in size, and the earliest branch representatives are usually tiny. The sister groups to the winged insects include the minute tardigrades (mostly less than 6mm), while the microscopic tardigrades or “water bears” are the sister group to the arthropods (including spiders and crustaceans ) and velvet worms.
The earliest mammals and some of the earliest dinosaurs (eg Eoraptor at less than two meters long) are also relatively small compared to their later, often huge, cousins.
Why bother getting bigger at all?
Being bigger has many benefits. Larger size may make it easier to evade predators (elephants and whales have few enemies other than humans), hunt prey, compete with competitors and suffer temporary hardships.
Large organisms tend to be better at conserving heat (due to their relatively smaller surface area) and have a greater capacity for information.
But scientists believe there is an upper limit to cell size. The mechanics of the cell division breaks down at very small and very large sizes.
All living things must also contend with a universal physical constraint noted by Galileo Galilei. Larger cells tend to have less surface area per unit volume. This means that the natural movement (diffusion) of gas, nutrient and waste molecules out of a cell is not enough to keep things going without a transport system. These molecules also have to travel further in larger cells.
So building a larger organism involves two things. First, group many cells so they can work together. Secondly, by making different cells specialized for different jobs – including structural support, digesting food and moving things like oxygen and CO₂ around.
The other option is to be flat or stringy (like horse worms) or thin and flat (like flat worms). 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.
The paleontologist Edward Cope (1840-1897) suggested that individuals within each lineage increased in size over evolutionary time. While this is true in a statistical sense, there are many exceptions, and mass extinction events often reset things to the smaller end of the spectrum.
Plot the distribution of sizes for almost any large group of animals and you will find a strikingly positive skew: most species are much closer to the smallest size than the largest size within their parent group, and there are very few large species. For example, there are more species of insects (about 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 body length of about 6mm. Giants such as the Hercules (17cm long) and the elephant (13cm long) beetles are extremely rare.
Small size allows animals to occupy more niches, and partition resources more finely, packing more species and individuals into the same habitat space. Insects are masters of this strategy.
The gentle will inherit the Earth – and beyond
Despite the tendency of organisms to evolve to larger sizes, the simplest and smallest organisms still have many incredible abilities that larger organisms lack.
Many of these reductionist “extremophiles” can live in environments that destroy most other forms of life.
Some archaea (single-celled organisms without nuclei) can withstand temperatures above 200°C around deep-sea vents, while other species can thrive in waters of high salinity, acid and alkaline concentration. Similarly, the tiny animals tardigrades can withstand temperatures between 150°C and -200°C, the vacuum of space, desiccation for years, and doses of radiation 1,000 times higher than those needed to kill a human. .
Read more: The secret world of moss, the ancient ancestor of all plants and vital to the health of the planet
Even tiny nematode worms are able to survive under two miles of solid rock.
Some scientists think that microbes could survive on interplanetary journeys within dregs. Scientists also think that any life we find elsewhere in the Solar System may have a common origin with life on Earth – starting small.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Matthew Wills has received funding from the BBSRC, NERC, The Leverhulme Trust and The John Templeton Foundation.
Tim Rock does not work for, consult with, own shares in or receive funding from any company or organization that would benefit from this article , and has disclosed no relevant affiliations beyond his academic appointment.
