Nine years ago, a very small, extinct human species was discovered on Flores in Indonesia:Homo floresiensis. Is this one meter tall 'hobbit' really as strange as it seems at first glance? The other fauna on this island answers this question.
In 2004, one of the strangest islanders ever was presented to the world:Homo floresiensis , also affectionately referred to as 'hobbit'. This new, tiny human species was found in the Liang Bua cave on the Indonesian island of Flores. With an estimated length of 1 meter and a brain size of 417 cc, this contemporary of our own kind challenges many ideas about human evolution.
Separate species?
Not everyone is convinced that this is a separate species. Some scientists suggest that H. floresiensis is nothing more and nothing less than modern humans, suffering from various disorders such as microcephaly (congenital defect in which the brain and skull remain small), cretinism (lack of iodine) or Laron syndrome (insensitivity to growth hormones).
In contrast, according to others, the remains resemble primitive early hominids from Africa. Throughout the discussion surrounding the status of H. floresiensis however, it ignores the fact that H. floresiensis was not an isolated, weird outlier, but part of an entire ecosystem of dwarf elephants, giant rats, Komodo dragons and giant marabou storks. Studying this seemingly strange ecosystem is therefore crucial to understanding the evolution of H. floresiensis .
Excavations on Flores
Flores is one of the Lesser Sunda Islands in the south-east of Indonesia. This area is largely volcanic and has never been connected to mainland Southeast Asia or Australia. The Liang Bua cave is located in the western part of Flores in the Mangarai province, about 15 kilometers north of the city of Ruteng.
The very first excavations at Liang Bua were made by the Dutch Father Theodoor Verhoeven, who was stationed at Flores in the 1950s and 1960s and had a huge interest in the archeology of the region. In the summer of 1965, he dug several shallow test trenches in Liang Bua. He found stone tools, potsherds and a number of graves of modern humans, but time constraints forced him to stop.
It wasn't until years later that Liang Bua was dug again, this time by Indonesian and Australian archaeologists. Convinced that older remains must be found in Liang Bua, they dug deeper than Verhoeven had gone. Their efforts were rewarded when the 'hobbit layers' were encountered in 2003.
Old age
The sediment layers containing the remains of Homo floresiensis found are between 95,000 and 17,000 years (Late Pleistocene) old. On top of the 'hobbit' layers lies a thick layer of black volcanic ash, which forms a clear separation between the older layers and the younger layers above them.
The remains of the 'hobbit fauna' have so far only been found in the layers below the volcanic ash, and the disappearance of this fauna is believed to coincide with the volcanic eruption, which is dated to 19,000-17,000 years ago. The layers above the ash layer are younger than about 11,700 years and these only contain the remains of modern humans and introduced species such as pigs, rats and porcupines.
Island Evolution
Islands are often seen as laboratories where the balance of natural selection can be very different than on the mainland. This is because an island is isolated to some degree. To reach an island you have to be able to fly, float or swim, and that is not given to every animal and plant species. For example, elephants, hippos and deer are excellent swimmers, while carnivores such as tigers and lions are not keen on crossing water.
The absence of mammalian carnivores is therefore characteristic of many islands, and the absence of this group results in a lower predation pressure on the possible prey animals.
In addition, as isolation increases, newcomers are rarer and already established species can survive longer. Behold, the recipe for special evolutionary scenarios.
Island fauna
Fossils from older sites on Flores indicate that only a handful of animal species were present on Flores at any one time. Almost all species found are species that can travel considerable distances over water, be it swimming (elephants, monitor lizards), flying (birds, bats), floating (turtles) or 'rafting' (rodents, monitor lizards, humanoids).
Mammal carnivores, such as felines and wolves, for example, are completely absent. This combination of factors resulted in the evolutionary selection pressures on Flores being very different from those on the mainland, resulting in species undergoing changes in their ecology, anatomy, reproduction and distribution.
Body size
One of the most notable changes in island species is a change in body size, known as the Island Rule or Foster's Law. This states that within an island ecosystem there is a tendency for large mammals to become smaller (dwarfism) and for small mammals to grow larger (giant growth).
On the mainland, being large is advantageous because it offers resistance to predators and a better competitive edge for food, and is beneficial for traveling long distances. On an island with a limited area (and therefore a limited amount of energy) and no predators, the need for big is gone. It is then even more efficient to be smaller.
Elephants are the best-known examples of so-called insular dwarfism. Dwarf elephants have been found on the Californian Channel Islands, Japan, the Philippines, Timor, Sulawesi, Crete, Malta, Rhodes and Cyprus, among others. The most extreme case of dwarfism in elephants was found in Sicily, where a fossil dwarf elephant with a shoulder height of only 0.9 m was found. Dwarfism also occurs in hippos in Crete, Sicily and Cyprus, and deer in Crete, the Philippines and in Japan's Ryukyu Islands.
Different rules apply to small mammals. On the mainland, being small is an advantage to escape predators. In addition, with a smaller body size you are better off in cases where food is scarce and competition is high. On an island without predators and less intense competition, the selection pressures on a small body size disappear, with the result that mainly rodents increase in body size.
The fossilized remains of giant rats have been found on a number of islands in Southeast Asia, including Timor, Flores, Sulawesi and the Philippines, as well as several islands in the Mediterranean and the Caribbean. One of the most remarkable examples of insular gigantism in small mammals comes from the Miocene of Gargano (Italy) where a giant hedgehog formed the largest insectivore with a skull length of about 20 cm.
The fossil faunas of Flores show both cases of dwarfism and gigantism. Remains of four species of giant rats have been found in Liang Bua, of which only the largest (Papagomys ) still exist today. The similarities between the oldest fossil rodent from the older Flores sites suggest that the oldest species may have been the ancestor of the later species. Dwarfism is clearly visible in the elephant remains:comparisons of the molars and deciduous molars of the Liang Bua dwarf elephant indicate a reduction of 30% compared to the direct ancestor.
The changes in body size in island mammals are often accompanied by changes in the limbs, which affect locomotion and locomotion. Here too, the lack of predators is seen as one of the driving factors behind this process. In combination with the often greater elevation changes and rocky subsoil of volcanic islands in particular, stability is preferred over speed, and the legs of many island mammals exhibit anatomical adaptations for locomotion in a lower gear (low gear locomotion ).
Characteristic for this are the shorter legs and the intergrowth of some bones, which results in greater stability that costs less energy. We see such adaptations in the mouse-goat of the Balearic Islands and in the deer of the Philippines and Crete, among others. Liang Bua's dwarf elephant also appears to have a fused ulna and radius, indicating a life at a slower speed.
Teeth and senses
In addition to changes in body size and musculoskeletal system, island species also experience changes in teeth and senses. For example, herbivores on islands often show more ridges on teeth in addition to teeth that are higher and continuously growing incisors. These dental changes are adaptations to eating foods that contain more silica, such as grasses.
On the mainland, herbivores have their eyes on either side of the skull with a view of almost 180° for each eye. In the absence of predators, the orientation of the eyes changes from side to side with better stereoscopic vision (not unimportant on islands with large elevation changes). Altered eye orientation was associated with a nearly 50% reduction in brain size in the Balearic mouse goat compared to modern goats.
Smaller brain size was also found in Madagascar's recently extinct pygmy hippos. The brain size turned out to be even smaller than thought based on the scaling of body size and brain size. Nerve tissue is one of the most 'expensive' tissues when it comes to energy consumption, and smaller brains provide significant energy savings.
Hobbit
But what about Homo floresiensis ? It's hard to imagine that hominids would somehow not be affected by the same evolutionary processes.
Homo floresiensis has an extremely small length, estimated at 1.06 m, which is unprecedented for the genus Homo and more consistent with estimates for the African species Australopithecus afarensis and A. africanus. Based on the skull and the rest of the skeleton, scientists believe that H. floresiensis evolved from a larger ancestor through insular dwarfism due to prolonged isolation.
The best candidate for the role of ancestor is Homo erectus , simply because no other early hominin seems to have reached Southeast Asia. Gay erectus reached Java around 1.8 million years ago, and seems to have reached Flores soon after, judging by the stone tools from Mata Menge.
Recent research has shown that primates can also develop a smaller body size as a result of an island environment. A scenario where H. erectus, like the other large mammals, after arriving on Flores it became smaller due to dwarfism seems very likely. The decrease in body size of H. erectus to H. floresiensis is about 52% and is comparable to island primates.
An important argument against the hypothesis that dwarfism is responsible for the evolution of H. floresiensis, is that the brain volume found (417 cc) is smaller than expected on the basis of body size. However, studies of the brain size of other island mammals (pygmy hippos and mouse-goats) show that smaller-than-expected brain size is not uncommon.
The body proportions of H. floresiensis show an unusual mixture of long arms, relatively short legs, robust bones and – entirely in hobbit style – large feet. Some scientists interpret these features as evidence of a close relationship with Australopithecus , but ignore the fact that similar patterns are more common in island mammals, and have even been found in the Minatogawa people of Okinawa Island in Japan.
Conclusions
The fossil faunas of Flores consist of a select group of animal species that are found nowhere else. This has strongly influenced the evolution of the species on the island:there are various giants, dwarfs and anatomical adaptations to an island environment. Flores is therefore no different from other islands.
Homo floresiensis is not diseased, deviant, or exclusive, but was part of an island ecosystem. Its short stature, robust lower limbs, large feet and small brains are not unique but are part of a pattern shared with other island mammals on Flores and other islands. They are the result of evolutionary forces acting on insular species due to the isolated position of the island and the unbalanced composition of the fauna.
- Super-fast evolution on islands (Knowledge Link Article)
- Evolution (Knowledge Link File)
- Complex Evolution (Knowledge Link News)
- Super-fast evolution of the earth mouse (Knowledge link news)
- Giant rat found on volcano (BBC)
- Working group Pleistocene Mammals