Small really does seem to be beautiful in evolutionary terms. The , and may look impressive but these giants are vastly outnumbered by microscopic and single-celled algae and fungi. Small organisms are also ancient and incredibly resilient.
Authors
Matthew Wills
Professor of Evolutionary Palaeobiology at the Milner Centre for Evolution, University of Bath
Tim Rock
PhD Candidate in Biology, University of Bath
The first evidence of single-cell organisms dates from around , soon after the newly formed Earth had to emerge. Multicellular animals evolved , with bigger and more complex animals appearing a little over . For most of Earth’s history, the planet has been dominated by organisms no larger than the diameter of a single human hair.
Large animals tend to take , so they reproduce more slowly. While mice have a short generation time (how long it takes a newborn to grow up and give birth) of , elephants take closer to .
Large species tend to and may be less able to cope with longer-term changes in the physical and biological environment. Larger organisms also tend to . Nothing much bigger than a domestic cat survived the asteroid impact that wiped out the dinosaurs 66 million years ago.
Being very big requires much more specialisation and slower reproduction, and both reduce 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.
So it’s not surprising that many groups of animals appear to , and the earliest branching representatives are typically quite tiny. The sister groups to the winged insects include the minute (mostly less than 6mm), while the microscopic tardigrades or “water bears” are the sister group of the arthropods (which include spiders and crustaceans) and velvet worms.
The earliest and some of the earliest dinosaurs (such as at less than two metres long) were also relatively small compared to their later, often gigantic cousins.
Why bother getting bigger at all?
There are many advantages to being bigger. Larger size may make it easier to evade predators ( and have few enemies other than humans), , outcompete rivals and endure temporary hardships.
Larger organisms also tend to be better at conserving heat (because of their relatively smaller surface area) and greater .
But scientists believe there is an upper limit on . The breaks down at very small and very large sizes.
All living things must also contend with a universal physical constraint noted by . Bigger cells tend to have less surface area per unit of volume. This means that the natural movement () of molecules of gases, nutrients and wastes in an out of the cell isn’t enough to keep things running without a transport system. These molecules also have further to travel in larger cells.
So building a bigger organism involves two things. First, grouping lots of cells so they . Second, making different cells – including structural support, digesting food and moving things such as oxygen and CO₂ around.
The alternative is to become flat or (like horsehair worms) or (such as flatworms). These animals don’t need an internal transport system because none of their cells (or their contents) are far from the surrounding air or water.
The palaeontologist proposed that individuals within all lineages tend to increase in size through evolutionary time. While this is true in a statistical sense, there are , and mass extinction events often reset things to the smaller end of the spectrum.
Plot the size distribution for nearly any major 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 relatively few big species. For example, there are more species of insects (around 5 million) than all other groups of animals put together, making them arguably the most successful animal group on Earth.
Most insects are beetles, with a . Giants such as the (17cm long) and (13cm long) beetles are extremely rare.
Small size allows animals to live in a greater diversity of niches, and to partition resources more finely, packing more species and individuals into 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 have many incredible abilities that larger organisms lack.
Many of these diminutive “extremophiles” can survive environments that wipe out most other forms of life.
Some archaea (single-cell organisms without nuclei) can withstand temperatures over 200°C around , while other species can thrive in waters of . Similarly, the tiny animals can withstand temperatures between , the , drying out for decades, and 1,000 times those needed to kill a human.
There are even tiny nematode worms able to live under .
Some scientists think that microbes could survive inside meteorites. Scientists also think any life we find might have a common origin with life on Earth – starting out small.
Matthew Wills has received funding from BBSRC, NERC, The Leverhulme Trust and The John Templeton Foundation.
Tim Rock does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
