Ancient organisms originally grew bigger to boost the distribution of their offspring
Long before any mammals, dinosaurs or even fish existed on Earth, the most advanced forms of life looked a bit like single fern leaves growing along the sea floor. These organisms, called rangeomorphs, literally stood out among more primitive life forms, growing larger and taller than most other life at the time. It’s easy to dismiss this difference as bigger being better, but that vague assertion doesn’t really provide insight into why these organisms would have ever started growing larger in the first place.
If not for the moving water of the ocean, the world of a rangeomorph would be pretty dull. Fossils of these 600 million-year-old organisms show no hints of mouths, organs or any anatomy that would enable mobility. They could apparently stand passively on the ocean floor, soaking up nutrients from the water around them without fear of being eaten or disturbed. This may sound a bit like plants growing in a modern forest, but that’s not really an accurate comparison- a sapling, for instance, will have to race its neighbors for access to sunshine, send out roots for water and soil nutrients, and avoid being killed by herbivores and parasites. As far as the fossil record shows, rangeomorphs worried for none of these things, making their varying sizes even weirder.
Size for the sake of their offspring
Fortunately for paleontologists, the lack of activity in the Ediacaran-era ocean has allowed entire communities of rangeomorphs to be preserved as fossils. This allowed researchers to not only compare sizes of each organism, but also the distributions of where each stalk grew. Once competition for resources or defensive positioning were eliminated, researchers took another look at where the tallest stalks grew in relation to their larger ecosystem.
What they realized is that being taller apparently helped rangeomorphs distribute their spores, suckers or whatever form of propagule they depended on to reproduce. By growing taller, individual rangeomorphs could reach slightly faster currents in the water that would then carry offspring out across a larger range of territory. Essentially, growing taller helped these rangeomorphs spread out faster than their shorter kin.
Source: Why life on Earth first got big, Phys.org
