In a confined space, evolution sets off in some unusual directions.
Life behaves in very distinctive ways on islands. Isolated from the mainland, evolution can lead in seemingly odd directions. As in Alice’s Wonderland, some species get very big, and some get very small. Obscure species can take centre stage, while normally common groups are completely absent. Some of the most iconic animals in the world — dodos, Komodo dragons, lemurs — have been shaped by island ecosystems. Heck, the whole native fauna of Australia is essentially a product of the creative whims of island evolution.
But what are the factors that contribute to these experiments in evolutionary isolation? And if put in the same situation, would humans, just another type of mammal, be susceptible to similar effects?
Our earliest understanding of island biogeography — the study of why particular species occur where they do — emerged in the early 20th century thanks to Baron Franz Nopcsa von Felső-Szilvás, a fascinating figure in his own right. This pioneering work was all the more impressive knowing that Nopcsa reached his conclusions from studying, not living ecosystems, but an assemblage of fragmentary dinosaur fossils. Nopcsa was an Austro-Hungarian aristocrat who became fascinated with palaeontology as a teenager, after his sister found some dinosaur fossils on one of the family estates. He studied geology at the University of Vienna and was particularly interested in the Cretaceous dinosaur sites discovered near his home in Transylvania. Nopcsa was one of the first researchers to study dinosaurs as if they were living animals that interacted with each other and their environment, and is considered the father of what we now call palaeobiology.
The dinosaur fossils from Nopcsa’s sites in Transylvania were smaller than expected, although apparently from adult animals. Nopcsa described Magyarosaurus in 1915, based on fossils from the Sânpetru Formation in what is now Romania. It was a member of the sauropod clade, the group in which we find record-breaking giants such as Brachiosaurus, Diplodocus and Supersaurus. But an adult Magyarosaurus was barely larger than a domestic horse. Based on the geology of his site, Nopcsa deduced that the rocks were laid down on a large island, and that the small size of the dinosaurs was caused by the limited resources within a confined environment. Over time, it was clear that Nopcsa’s brilliant insights applied to all island ecosystems and were not peculiar to his fossil sites.
But there is more to life on islands than Nopcsa’s singular observation. Life on isolated islands has been described with the ‘island rule’: Large species get smaller, yes, but small species get larger. The extremes are the easiest to grasp. During the Pleistocene period (2.588 million to 11,700 years ago), the islands of the Mediterranean supported populations of dwarf elephants, some not much bigger than a Shetland pony. Less space means fewer resources, and large size becomes disadvantageous in such an environment. Conversely, a small species like, say, a pigeon, when left to its own devices on an island, might bulk up a little and lose the power of flight: Why bother with that energetic expense when there are no predators on an isolated island? Before you know it, they’ve evolved into the dodo.
Another significant rule on islands is the founder effect. Understandably, the gene pool in an island population is going to be limited, and so the fate of its entire future genetic stock rests on the quirks of the individuals who get there first. This concept is creatively deployed in Kurt Vonnegut’s 1985 science-fiction novel Galápagos, in which a group of people are shipwrecked on a fictional Galápagos island and remain there indefinitely after a global catastrophe. One of the women gives birth to a daughter, born completely covered with fur. Over many generations, this normally rare condition becomes dominant in the population, and, after a million years, results in the evolution of a hair-covered, seal-like human species.
Every island ecosystem contains species that are fundamentally shaped by the various rules of evolutionary isolation. But the species on isolated islands aren’t randomly selected: Certain groups are more likely to migrate to islands than others. Animals that can fly — birds, bats, insects — are the most obvious candidates. It is theorised that many terrestrial animals have made it to islands by travelling on rafts of vegetation, pumice and other flotsam. Reptiles, with their low metabolism, are particularly well suited to long sea voyages on which food may be scarce or absent.
Of course, there is one vertebrate that has spread across the globe into every conceivable habitat, no matter how inaccessible: humans. (As we’re going to be dealing with a few different kinds of humans, let’s call them ‘hominins’ — members of the genus Homo and their immediate ancestors.)
The question of how Homo might respond to the pressures of island isolation was answered in 2003 with the discovery of Homo floresiensis. Fossil remains of H. floresiensis have so far been found only on Flores, a small member of Indonesia’s Lesser Sunda Islands group. The fossils amazed scientists when they were found, as they seemed to be from a completely new kind of human that had survived until as recently as 12,000 years ago. This new human species became one of the biggest science stories in the popular media (and resulted in a Tolkien-derived nickname, which I’m not going to perpetuate here). H. floresiensis was very short, probably just over a metre high on average, and had a strikingly small brain.
This new hominin was so bizarre that many researchers were convinced that its features were the result of pathology, rather than being a distinct human species. H. floresiensis’s unique morphology has been attributed to a plethora of growth defects, diseases and malformations. But the recent discovery of a second, significantly older group of H. floresiensis-like fossils at a different site on Flores would seem to conclusively disprove these hypotheses.
H. floresiensis appears to be a poster child for island dwarfism, although its evolutionary history is not yet well understood. Two origin stories present themselves: Either H. floresiensis evolved from small ancestors and stayed small, or it evolved from larger hominins and was dwarfed by the evolutionary forces of an isolated island.
In the first scenario, the small ancestors are suggested as Homo habilis, a diminutive early human, or a species of Australopithecus — upright walking proto-humans like ‘Lucy’. With their short stature and small brains, these species are similar to the hominins of Flores and morphological studies support a close relationship. But these proposed ancestors are known only from fossils found in East Africa. If this theory is correct, it would require a dispersal of small hominins from the African continent about a million years earlier than is currently theorised in orthodox models of early human migration. No archaeological or fossil evidence has yet been found for such a migration.
An alternative ancestor for H. floresiensis was Homo erectus, an archaic human species about the same height as modern Homo. Unlike the smaller African hominins, we know that H. erectus was at the right place at the right time: H. erectus fossils are known from the Indonesian island of Java and have been dated at 1.2 million years old, just before the earliest hominin records on Flores from 700,000 years ago. Though these specimens are very fragmentary and can’t be definitively classified, they are thought to be an ancestral form of H. floresiensis. Until more fossils are found, the species’ evolutionary path will remain an incomplete puzzle.
Although H. floresiensis’s past is poorly known, there are currently plenty of island-living primate species that we can directly observe: the Natuna Island surili from Indonesia; the pig-tailed langur and Kloss’s gibbon from the Mentawai Islands, off the coast of Sumatra; and the toque macaque and purple-faced langur from Sri Lanka. In most of these cases, there is a nearby mainland relative from which it can be reasonably assumed the island population derived. Comparisons between them show that these primates are, indeed, significantly smaller than their mainland counterparts. The Natuna Island surili, for example, is about 26% smaller than its mainland equivalent, the banded surili.
Most of the islands with primate species were only created by the melting of ice after the last glacial maximum, less than 12,000 years ago, and yet they already boast their own distinct primate species. Just imagine what evolution could do with a handful of hominins beached on an island for several hundred thousand years.
Edited by Andrew Katsis