Art 'N Bones: Early evolution of small body size in Homo floresiensis

By: Yousuke Kaifu, Iwan Kurniawan, Soichiro Mizushima, Junmei Sawada, Michael Lague, Ruly Setiawan, Indra Sutisna, Unggul P. Wibowo, Gen Suwa, Reiko T. Kono, Tomohiko Sasaki, Adam Brumm & Gerrit D. van den Bergh

This paper talks about hominid bones unearthed in the So’a Basin in Central Flores, Indonesia. Fragments of a mandible and 6 different teeth belonging to a small hominin were discovered in the sandstone layer of the Mata Menge site. These fossils are dated to between 0.65 and 0.773 Ma. They share morphological similarities to H. floresiensis from Liang Bua (Western Flores) and H. erectus from Java. The fossils seem to be the ancestor of H. floresiensis and are theorized to be dwarf descendents of H. erectus.

Evidence of this link is that the mandible and teeth found at the Mata Menge site are smaller than those found in H. floresiensis from Liang Bua. This hints that these bones may have existed more than 600,000 years before the oldest H. floresiensis fossils found. Unfortunately, due to a lack of cranial bones found in the Mata Menge, there is limited data on body size evolution on Flores. This paper analyzes an adult humerus found later in the same Mata Menge site and concludes that it is morphologically more similar to a Homo species such as H. naledi rather than an Australopithecus. A molar crown also shows similarities to H. erectus rather than early African Homo. These analyses support the fossils being classified as early H. floresiensis. It also provides evidence that between -1.0 and 0.7 Ma, the ancestor of H. floresiensis went through a drastic size reduction from its larger bodied ancestor H. erectus.

More specific data illustrates the smaller body size of H. floresiensis through the discovered humerus and teeth. Scientists believe the humerus from specimen SOA-MM9 belongs to an adult due to its cortical bone histomorphology. Cortical samples were taken from the mid-posterior shafts of the humerus and compared to a modern human sample. The Osteon Population Density (OPD) and Haversian Canal Index (HCI) of the fossil (OPD = 16.5, HCI = 0.85) were greater than any modern human subadult humerus (OPD = 0.0-8.9, HCI = 0.0-0.63). This indicates that the bone specimen came from a mature adult, not a child. Conditions like osteogenesis imperfecta, which results in a shorter stature, could be ruled out since those with the condition display lower than normal OPD values. The humeral size of the Mata Menge fossil is smaller than any other adult individual of small-bodied fossil hominins such as H. naledi. The Mata Menge humerus’s minimum circumference is 46 mm which is less than the smallest humeri for prehistoric humans (46.5 mm). Its centroid size is also the smallest compared to any known adult specimen of Australopithecus, Paranthropus, H. naledi and H. floresiensis.

In the end, researchers concluded that there were at least 4 individual’s remains found in the Mata Menge site, 1 adult, 1 adolescent/ young adult, and 2 children. All 4 of these specimens displayed an extremely small body size for their age. This shows that small body size was not an individual characteristic, but rather a population characteristic. Possible theories to explain the small stature could be due to a lack of predators. Flores was so isolated that there were no natural predators for early H. floresiensis. Having a larger body size would be a disadvantage evolution wise as larger bodies require more energy and food to sustain itself.