image: This graph shows the cranial capacities in cubic centimeters for 118 fossil hominids plotted against time, extending back almost 3.5 million years. The arrow indicates the highly incongruous value reported in Nature by Brown et al. in 2004 for Homo floresiensis, described as an insular dwarf derived from Homo erectus. The relatively tiny brain size only 18,000 years ago does not fit into known patterns of hominid brain size and development. It is "off the chart." view more
Credit: Graph by Robert D. Martin, Courtesy of The Field Museum
Increasingly, however, this controversial conclusion is being questioned. In a Technical Comment to be published in the May 19, 2006, issue of Science magazine, scientists led by Robert D. Martin, PhD, Field Museum Provost and world-class primatologist, say that the bones in question do not represent a new species at all. A far more likely explanation is that the bones belonged to a modern human who suffered from microcephaly, a pathological condition that causes small brain size, often associated with short stature.
The proposed new hominid species is based primarily on a specimen known as LB1 consisting of a diminutive adult skull and partial skeleton about three feet tall. Initially, H. floresiensis was claimed to be a dwarf derived from Homo erectus ("upright man"), a human ancestor that lived as far back as 1.8 million years ago. This seemed like an appealing explanation because islands are known to play tricks on the evolution of animals, sometimes causing them to shrink due to limited food supplies and the reduced presence of predators.
But all mammals that have shrunk due to the constraints of an island's resources – or any other reasons, such as selective breeding – have done so within certain parameters: Body size can shrink considerably, but brain size always shrinks moderately. At 400 cubic centimeters LB1's brain is simply too small to follow this universal law. In fact, for LB1 to be a dwarfed form of H. erectus, it would have to have been just one foot tall with a body weight of only four pounds to explain such a diminutive brain. "The tiny cranial capacity of LB1, which is smaller than in any other known hominid younger than 3.0 million years old, is demonstrably far too small to have been derived from Homo erectus by normal dwarfing," Dr. Martin says.
Series of problems
Small brain size is just one of several problems with the science behind claims that LB1 represents a new species, according to Dr. Martin and his colleagues. The primary problem, clashing directly with the tiny brain size, is the sophisticated nature of the stone tools found in the same cave deposits where the fossils were discovered. Based on their size, style, and workmanship, these tools belong to types that are consistently associated with modern humans, or Homo sapiens, according to James Phillips, PhD, Adjunct Curator of Anthropology at The Field Museum, Professor of Anthropology at the University of Illinois at Chicago, and co-author of the Technical Comment. Such tools have never been associated with H. erectus or any other early hominid, he says. "These tools are so advanced that there is no way they were made by anyone other than Homo sapiens." Another problem with the science surrounding the interpretation of the Flores fossils is that a distinct species of hominid so closely resembling modern humans but living only 18,000 years ago is inconceivable given that H. sapiens had almost certainly reached Flores by that time, according to Dr. Phillips.
A main point in the Technical Comment by Dr. Martin and his colleagues is that the evidence used by some scientists to rule out the possibility that LB1 could have been microcephalic is flawed. In research published last year that attempted to exclude this possibility, a team led by Dean Falk, PhD, studied a virtual brain cast from a single microcephalic skull – even though microcephaly can take dozens of different forms.
Furthermore, the skull from which the brain cast was made was that of a 10-year-old child, whereas LB1 was an adult and should have been compared with human microcephalics with a relatively mild condition that would have permitted survival into adulthood. Finally, the skull from which the virtual brain cast was generated is a poor-quality, plaster copy comprised of two parts that do not match up! The plaster copy of the microcephalic skull, which was made more than 100 years ago, is housed at the American Museum of Natural History in New York. It took some detective work on the part of Dr. Martin to track down the original skull (from which the plaster copy had been made). It is held in the collections at the Natural History Museum in Stuttgart, Germany.
"This defective plaster copy of a microcephalic skull used in the study by Falk et al., published in Science March 4, 2005, is inappropriate for any scientific study, especially one dealing with a topic as demanding and high-profile as this one," Dr. Martin says. "Quite simply, it was the worst possible choice for this study. The cranial capacity turned out to be only 260 cc, just over half of that recorded for the LB1 skull, and is one of the smallest that I have so far found in a survey of over 100 human microcephalics."
This leaves the theory that LB1 was a microcephalic modern human as the only plausible explanation for the Flores fossils, according to Drs. Martin and Phillips and their colleagues.
"There has been too much media hype and too little critical scientific evaluation surrounding this discovery, and it is simply unacceptable that papers should be published without providing proper details of the specimens examined," Dr. Martin says. "The principle of replicability is fundamental to good science, and it has not been respected in this case."
The other co-authors of the study are A.M. MacLarnon, PhD, School of Human & Life Sciences, Roehampton University in London; W.B. Dobyns, PhD, Department of Human Genetics, University of Chicago; and L. Dussubieux, PhD, and P. R. Williams, PhD, both of The Field Museum. Dr. MacLarnon is a specialist on size changes in the central nervous system, and Dr. Dobyns is a leading authority on genetic aspects of the human nervous system. Meanwhile Drs. Dussubieux and Williams conducted elemental analysis on samples from the two halves of the Stuttgart microcephalic skull.
A full-length manuscript by the main authors raising these same issues but in much greater detail will soon be submitted for publication in The Anatomical Record.
Digital images available:
On the left is the LB1 skull, found on the Indonesian island of Flores. It is the basis for claims that the Flores fossils represent a new species of hominid, or humanlike primate. Note, however, that this skull is very similar to the one on the right (from the Royal College of Surgeons in London) of a modern adult human who suffered from microcephaly. The close similarity between these two skulls, which are drawn to the same scale, indicates that the LB1 skull from Flores could well be that of a modern human microcephalic rather than representing a new hominid species.
Drawing by Jill Seagard, Courtesy of The Field Museum
Skull cast and cast of the endocranial cavity (endocast) from the Royal College of Surgeons in London of a modern adult human who suffered from microcephaly. It is strikingly similar to the following skull and endocast of a 32-year-old microcephalic woman. Together, the two specimens provide evidence that the LB1 skull from Flores, which is so similar to these two specimens, could also have been a microcephalic adult.
Photo by John Weinstein, Courtesy of The Field Museum (Negative # Z94438_07Ad)
Skull cast and cast of the endocranial cavity (endocast) from a 32-year old woman who had the body size of a 12-year-old child. She lived in Lesotho, a county in Southern Africa, and these casts are part of The Field Museum's collection (Accession Nos. A219679 And A219680). This specimen and the one above have a relatively normal exterior appearance despite their very small size. Together they demonstrate that the LB1 skull from Flores could also have been an adult who suffered from microcephaly.
Photo by John Weinstein, Courtesy of The Field Museum (Negative # Z94438_01Ad)
This graph shows the cranial capacities in cubic centimeters for 118 fossil hominids plotted against time, extending back almost 3.5 million years. The arrow indicates the highly incongruous value reported in Nature by Brown et al. in 2004 for Homo floresiensis, described as an insular dwarf derived from Homo erectus. The relatively tiny brain size only 18,000 years ago does not fit into known patterns of hominid brain size and development. It is "off the chart."
Graph by Robert D. Martin, Courtesy of The Field Museum
Robert D. Marin, PhD, Provost and Curator of Biological Anthropology at The Field Museum, has devoted his career to studying primate development and evolution. In his quest to achieve a reliable reconstruction of primate evolutionary history, Dr. Martin has studied an extensive array of characteristics in the living species, including anatomical features, physiology, chromosomes and DNA.
Dr. Martin has been particularly interested in the brain and reproductive biology, as these systems have been of special importance in primate evolution. With skeletal features, it is possible to include the fossil evidence and thus to include geological time in the picture. By studying living primates in the field in the forests of Africa, Madagascar, Brazil and Panama, Dr. Martin has also been able to include behavior and ecology in an overall synthesis. That synthesis was first presented in his textbook Primate Origins and Evolution, published by Princeton University Press in 1990. Since then, he has been working on refinements in several different directions.
Photo by John Weinstein, Courtesy of The Field Museum (Negative # GN90075_36Ac)
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