发布时间: 2019-11-10 22:55:27|报码现场直播一期 来源: 威客网 作者: 段路路


  Back in 2003, the skeletal fragments of nine small hominids were found in the Liang Bua Cave on Flores, an Indonesian Island. The find included an almost complete skull and partial skeleton of a probable female around 30 years of age, which was named LB1. Once reconstructed, researchers were shocked to find that LB1 was only three and a half feet tall.

  Nicknamed The Hobbit, LB1 was a part of a new species called Homo floresiensis that is believed to have survived until 60,000 years ago, which makes it still contemporary with Homo sapiens in that region. Many in the scientific community, however, criticized the decision to name LB1 as part of a new species, saying that she was simply an abnormal Homo sapiens. That theory flew out the window when another hobbit-sized skeleton on Flores was discovered, one even older than LB1. And just this year, another dwarf human species--dubbed Homo luzonensis--was discovered in the Philippines.

  Now, researchers think they know why these Hobbit-like hominids were so small--they evolved much faster, thanks to being stuck on an island.

  As it often happens with evolution, its a matter of opportunity, Pasquale Raia, associate professor of paleontology and paleoecology at the University of Naples Federico II, told Fox News. Islands are different [because] they are small, they harbor little diversity of species, and more importantly, you cant go away.


  In their paper published in Biology Letters, study leader Jos Alexandre Felizola Diniz-Filho of the Federal University of Goias, Raia and collaborators point to the island rule originally proposed by zoologist J. Bristol Foster in 1964. Foster observed that when large-bodied species moved to an island, they would shrink as they evolved, eventually leaving dwarf descendants. When a smaller species moved to an island, the opposite was true, and they would evolve into larger species. Examples of the island rule include extinct donkey-sized hippos in Cyprus and the giant rats that were once found in the Caribbean and huge insects that are found in New Zealand.

  Biologists believe this is due to a variety of factors. For one, being larger helps fight off natural predators, of which there may be fewer (or even none) of on an island. There could also be fewer resources to survive on, making it easier for smaller species (that require less sustenance) to thrive while larger species would have to adapt. There may be fewerspecies to compete for food with as well, enabling smaller animals to reproduce faster and eat more.

  If you are able to survive on an island, [the] chance is that you will become a dominant species there (populations that do not fare well simply go extinct on islands) and with so little competition from other species to bother with, your main goal will be producing more offspring, Raia said. Since small individuals reproduce faster and the smallest among their offspring will leave more descendants, the process accelerates generation after generation.

  Raia and Diniz-Filho theorized that thanks to the island rule, Homo floresiensis--which is likely descendant of the larger Homo erectus--reproduced quickly and evolved at a very fast rate. Judging by the geologic history of Flores and the oldest recovered Homo floresiensis fossils, the researchers had estimated that the species eventually became hobbit-sized in about 5,000 years.


  To find out how long it took for Homo floresiensis to attain its small size (or how fast its evolution was, in other words) the researchers developed a computer model that could simulate body size evolution in all sorts of different biological and ecological set-ups. In the model, the body size of Homo floresiensis is the end product, and the larger body size of its ancestor, Homo erectus, the starting point. The computer model then replicates history.

  At some time (around one million years ago in the real world), a few Homo erectus individuals reach the island of Flores and start living and reproducing there, Raia said the model assumes. Generation after generation, the smallest among these colonizers, leave a bit more descendants, on average, than their siblings, and so on.

  Offspring in the model would inherit either larger or smaller-sized body genes, with optimum body-sized populations reproducing more (optimumbody size is decided upon by the islands resources at the time). The model calculated that the desperate drive for these hot and heavy hominids to reproduce slowed down with each subsequent generation as the body size of the species shrinks.

  The strength of this drive--the greater reproductive success of the smaller individuals--diminishes as the body size decreases from one generation to the next, Raia explained. We counted how many generations it takes to reach the endpoint, and found that some 350 [generations] were enough, which translates to some 5,000 years.


  Raia notes that his description is a very simplistic explanation of the computer model, but its what it does in practice. The model ran 10,000 simulations with random parameters and variables.

  In the real simulations, the model is full of complicated specs: the available resources change as climate changed, new colonizers may reach Flores igniting new and larger offspring into the system, and more importantly the process is simulated with realistic parameters in terms of genetic heritability, he said. To make a simple example about this latter point, although a pair of tall parents will tend to have a tall child, this is only true on average, because the heritability of human stature has both a genetic component and environmental component, our model includes both.

  In their paper, the researchers write that their findings indicate fast evolution can happen under the right circumstances and that natural selection could play a large role in body size on islands.

  One thing that still mystifies Raia and Diniz-Filho is how these hominids, with tiny, chimp-sized brains, thrived for so long on the islands. LB1, for instance, lived between 60,000 and 90,000 years ago, while her other counterpart on the island lived 700,000 years ago.


  Perhaps we are overestimating the importance of brain size to survival to begin with, yet it is still astonishing to me they stood there on Flores for some 1 million years with their tiny bodies and little brains, successful as nobody could possibly fathom, Raia said. I wish we will in the future be able to give a deeper look into their brain functioning by studying the shape and proportions of the different brain cortical areas.

  Diniz-Filho agreed, telling Fox News that hed not only like to improve the computer model to study on brain/body size and structure but to focus more on potentially influencing outside factors as well.


  Well try to explicitly model how island characteristics and demographic characteristics of the population would drive the evolution of such attributes associated with [the] dwarfing process, he said.









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