Biogeographically speaking, islands are in some ways just small bits of mainland. Indeed, sometimes islands are literally small bits of mainland. They have broken away from the edge of a continent and moved out to sea, propelled by plate tectonics. Alternatively, they are still strongly connected to the mainland, but the connection happens for the moment to be flooded by high sea levels. Between ice ages, sea levels are high, as they are today. During ice ages, the water gets converted to ice, and sea levels drop. At the height of the last glaciation, roughly twenty thousand years ago, sea levels were over a hundred meters lower than they are now. If global warming does not prevent the return of the next glacial period, sea levels will then fall as in previous ice ages, and many coastal islands will return to being part of the mainland. Britain and Ireland will no longer be islands but, instead, simply the western edge of Europe, as they were during the last ice age.
That could be a pity, because in many ways, islands are special. Cut off by water, isolated from contact with others of their kind, island animals and plants can go their own way evolutionarily. The result is many species unique to a particular island and found nowhere else. The lemurs of Madagascar are a classic case for primates. The nene, or Hawaiian goose, is classic for birds.
A relevant piece of biogeographic jargon out there is the word “endemism,” or “endemic.” It means confined to a particular region. The lemurs are endemic to Madagascar. One often thinks that endemism means confined to a small area. However, the size of the region is irrelevant. In Hawaii, some bird species are endemic to just one island, but the kangaroo is endemic to Australia, and the reindeer is endemic to northern Eurasia.
Biogeographers speak of island “rules” to describe and explain a number of general ways in which island fauna and flora differ biologically from mainland fauna and flora. Humans follow some of the rules, but not others. To introduce one of the island rules, I am going to start with “Flo,” the Flores island “hobbit.” Strictly, Flo is probably not relevant in a book on human biogeography, for reasons that I will come to. However, the story allows me to introduce an island rule that Pacific-island humans break. Understanding why a phenomenon breaks a rule is often just as informative concerning underlying causes as understanding the reason for the rule in the first place.
In 2004, the announcement in the journal Nature of a new species of the genus Homo from Asia, Homo floresiensis, electrified the anthropological world. The species name comes from the discovery of 40,000- to 13,000-year-old bones in a cave on the island of Flores, east of Java and Bali in the Indonesian archipelago.
“Flo,” or the “hobbit,” as the tiny new species affectionately became known, was the first new hominin species claimed in Asia for over a century. The last one, oddly enough, was the first Homo erectus ever discovered. Named Java Man, it was found in 1891 by the Dutch paleoanthropologist Eugène Dubois or, to give him his impressive full name, Marie Eugène François Thomas Dubois.
The hobbit was strange in many ways. Apparently alive on Flores well over ten thousand years after the last non-human hominin died out elsewhere in the world, its skeleton turned out to be weird enough that doubts immediately arose regarding almost everything to do with it. Arguments and counter-arguments flared, burned out, and flared again. And the arguments were not, sad to say, always politely expressed, reasoned differences of opinion. Sarcastic commentary with the word research in quotation marks to denigrate the others’ scholarship marked one exchange. Other different opinions were characterized as “unsubstantiated assertions” by a “vocal group.” Suggestions of, shall we say, incomplete reporting of measurements, along with accusations of mishandling of the original material and denial of access to it, exemplify just some of the all-too-human nature of the early debate.
If the hobbit had been only slightly different from all human ancestors, probably only paleontologists, a journalist or two, and the occasional informed member of the public would have paid any attention. But the hobbit was extraordinary. It had such a mix of modern, ancient, and its own unique features, such as almost ridiculously long feet, that disagreement was inevitable.
Perhaps the most amazing feature of the hobbit was its small size—hence the nickname. Let me say that only one near-complete skeleton has been found, so I refer to the hobbit in the singular. The hobbit stood just over one meter high. That is thirty centimeters shorter than any human pygmy. I am a normal-sized male for my generation in the UK, at one meter seventy-eight centimeters. The hobbit would reach the bottom of my rib cage. The hobbit was stocky, so at thirty kilograms it weighed more than many modern-day pygmy adults do. Added to its amazingly small stature is an amazingly small brain. The one skull found shows a brain so tiny—four hundred and twenty-five cubic centimeters, one third the size of a modern human’s, in fact the size of a chimpanzee’s—that we have to go back three million years in human evolution, beyond Homo to the australopithecines, to find another hominin with a brain this small.
Is the hobbit a relict australopithecine species? If not, what is it? How do we explain its extremely long feet? How can the hobbit be a hominin, and yet have so small a brain? Why is this hobbit so small? How do we explain its mix of modern and ancient traits?
With no evidence of any australopithecines outside of Africa, one of the early explanations for the hobbit’s small body size, and especially its small brain size, was that it suffered from microcephalic dwarfism. The “microcephalic” part of that is just the typical doctor’s Greek jargon for “small-brained.” I cannot resist a story from my brother-in-law. He had a swollen knee. He went to the doctor. “Aah,” said the doctor, “you have patellitis.” Patellitis is Latin and Greek for swollen knee!
Other pathologies to explain the hobbit are Laron syndrome, or a form of cretinism. Both of these conditions come with small stature. The first is a genetic condition, while the second can result from mineral deficiency, especially lack of iodine. It characterizes individuals who lack fully functioning thyroid glands, the glands that produce hormones essential for full growth. A recently suggested pathology to explain the hobbit’s features is Down syndrome, which can explain the odd mix of modern and apparently ancient traits.
If the hobbit was in fact a modern human suffering from a disease, it would not be relevant to this book on the biogeography of humans, given that none of the suggested diseases to explain its small stature and brain size confine themselves to any one part of the world. However, the hobbit lived on the small island of Flores, in eastern Indonesia, and the biogeography of small islands is highly relevant to the hobbit. Conversely, the hobbit is relevant to the biogeography of small islands, given how intensely scientists have studied it.
Flores covers on the order of thirteen and a half thousand square kilometers, approximately the size of Connecticut in the USA, or Northern Ireland in Britain. A feature of small islands is that species that are large-bodied on the mainland often evolve to become smaller, sometimes far smaller. On the islands of the Mediterranean Sea, such as Cyprus, Malta, Crete, and Sicily, elephants and mammoths shrunk over many generations’ time to half the height that they were on the mainland, so they ended just one and a half meters high at the shoulder.
California’s Channel Islands too had their own pygmy mammoth, just a little taller than the Mediterranean elephants. Similarly, the mammoth of Wrangel Island off the north coast of Siberia was roughly thirty percent smaller than the average mainland mammoth, as judged from their tooth sizes.
Flores is no exception to this phenomenon of miniaturization. An extinct form of elephant in mainland Asia, stegodons, were some of the largest elephants ever to have lived. The Flores version on the island at the same time as the hobbit was one third smaller than its mainland relative. The adults, at about eight hundred and fifty kilograms, were too big for the hobbit to hunt, but they hunted the young ones. Archeologists have inferred this from cut marks on the bones in the cave.
~~Humankind: How Biology And Geography Shape Human Diversity -By- Alexander H. Harcourt
No comments:
Post a Comment