Oct. 24-26, 2009, Yingjie Overseas Exchange Center, Peking University, Beijing, China中文版

The evolution of mammalian brain - from Darwin's The Origin to the new evidence of Mesozoic mammals

Jin Meng

Division of Paleontology, American Museum of Natural History, New York, NY 10024, USA. jmeng@amnh.org

Abstract

In the year 2009, we celebrate both Charles Darwin’s 200th birthday and the 150th anniversary of the publication of On the origin of species (the Origin, Darwin 1859) by presenting various research results that make sense only within the context of evolution. While numerous new disciplines related to evolution have been established since Darwin, many classical researches have remained lively and made enormous progresses, of which is the study on brain evolution (Smulders, 2009a). Although from the first edition to the later ones of the Origin, Darwin did not discuss brain evolution in any detail, but he paid increasingly attention to the issue. He did state that natural selection applies to the brain as it does to all the other organs in the 6th edition (Darwin 1872). However, Darwin was aware of the issue not only because brain evolution was a subject actively studied by his contemporary anatomists, such as Huxley, but also because he had published the first edition of The descent of man (Darwin 1871) before the 6th edition of the Origin.

In the waves of debates inspired by The Origin, one of the key issues was the position of humans as part of the animal kingdom, an unavoidable subject in Darwin’s time. This issue was then best reflected by The descent of man, an argument for the similarities and continuity between apes and humans. Darwin wrote: "No one, I presume, doubts that the large proportion which the size of man's brain bears to his body, compared to the same proportion in the gorilla or orangutan, is closely connected with his mental powers." As other similarities between those of humans and other mammals, Darwin emphasized the importance of the brain: “It is notorious that man is constructed on the same general type or model with other mammals. All the bones in his skeleton can be compared with corresponding bones in a monkey, bat, or seal. So it is with his muscles, nerves, blood-vessels and internal viscera. The brain, the most important of all the organs, follows the same law, as shown by Huxley and other anatomists” (Darwin 1871).

Within the evolutionary framework, the human brain would be subject to the pressures of natural selection as those of other animals. Therefore, understanding what makes human brain, and functions and behavioral activities that go along with it, different from those of other animals requires exploring the brain evolution and variety of other animals, including those that have become extinct in the geological history (Jerison, 1973). A large portion of brain research has focused on describing and understanding the patterns of descendant with modified mammalian brains and asking the questions of when, what, how and why these patterns and modifications took place (Northcutt, 2003). Early works largely addressed the easy-to-measure aspects of brain morphology, such as the overall size or the sizes of major brain parts (Healy and Rowe 2007). While these works continue to be fruitful (Isler et al., 2008; Smulders, 2009), new efforts have been made to build hypotheses for brain evolution that integrate data ranging from molecular biology, neuroscience, ecology to paleontology (Aboitiz et al., 2003; Smulders, 2009). This is not only because we want to understand why we are and where we come from, but because even in a medical point of view studying brain evolution is relevant as long as biomedical researches use other animals to help us understand the human brain (Smulders 2009a).

Unlike other amniotes, the mammalian brain is characterized by the unique isocortex that is a 6-layered structure, differing from the 3-layered telencephalic laminar structures such as the hippocampal formation, the olfactory cortex, and the reptilian cortices (Supèr et al. 1998b). The development of the isocortex is externally characterized by expansion of the forebrain. This feature can be estimated from an endocast of the braincase in early mammals. However, fossil cranial endocasts in early mammals are rare and often distorted in preservation. One of the classic studies of brain morphology in Mesozoic mammals is from a braincase endocast of the Jurassic mammal Tricodonnodon (Simpson, 1927; Rowe, 1996), which has been widely cited as the morphotype representing the initial stage of brain expansion among the earliest mammals. However, my personal observation of the original specimen of Triconodon convinced me that the specimen has been squashed and that the reconstructed brain of Triconodon based on the distorted endocast may not be accurate. In addition to the rare preservation of braincase endocasts in Mesozoic mammals, studies on mammalian brain evolution often focused on the overall size of the brain alone, although the latter is one of the most important aspects in mammalian brain evolution. Because the brain is the information process center, it is meaningful to understand its evolutionary change in relation to changes of sensory organs, behaviors and ecology of mammals. With newly discovered specimens of Mesozoic mammals and CT-scan techniques we are now able to obtain some new data previously unavailable that permits a synthetic analysis of brain evolution in early mammals.

Here I present a preliminary study on the braincase of the Early Cretaceous triconodont mammal Repenomamus robustus (Wang et al., 2001). Four skulls of the species preserved in 3-dimention are CT-scanned using a High-Resolution X-ray Computed Tomography Facility in Austin, Texas. The digital endocasts were extracted from the CT scans of the skulls using the program VGStudio. Segment portions of the endocasts were rendered, representing distinct structures such as the nasal cavity casts, olfactory bulb casts, parafloccular casts, inner ear (cochlea and semicircular canals) and brain casts. In addition to the CT-scans, the eyeballs were reconstructed based on the shape and size of the orbits. Several observations can be made from the new specimens and CT data: 1) The brain of Triconodon may not be as big as it was reconstructed by Simpson. 2) The relatively size of the brain of Repenomamus can be estimated by its relationships with the olfactory bulbs and the inner ears. It is more developed than those of non-mammalian cynodonts, but less so than the Cretaceous theriiform mammal Vincelestes and therian mammals. 3) The nasal cavity and large olfactory bulb casts indicates that the sense of smell is probably most developed in Repenomamus, which could be the factor that caused the initial expansion of the brain. 4) The eyes and inner ears are more developed than in non-mammalian cynodonts but less so than in theriiform mammals. This suggests that expansion of the brain in theriifroms is probably related to enhanced functions of vision and hearing during mammalian evolution. 5) The enlargement of the brain must also be associated with other functions, such as mastication, because the morphological changes of the skull will affect the attachment and arrangement of the jaw muscles. Thus, expansion of mammalian brain during the evolution could also be coupled with the improvement of mastication.

References
Aboitiz, F., D. Morales, and J. Montiel. 2003. The evolutionary origin of the mammalian isocortex: Towards an integrated developmental and functional approach. Behavioral and brain sciences 26: 535–586.
Darwin, C. R. 1859. On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life, 1st edition. London, UK: John Murray.
Darwin, C. R. 1871. The descent of man, and selection in relation to sex, 1st edn. London, UK: John Murray.
Darwin, C. R. 1872. The origin of species by means of natural selection, or the preservation of favoured races in the struggle for life, 6th edition. London, UK: John Murray.
Isler, K., E. C. Kirk, J. M.A. Miller, G. A. Albrecht, B. R. Gelvin, and R. D. Martin. 2008. Endocranial volumes of primate species: scaling analyses using a comprehensive and reliable data set. Journal of Human Evolution, 55: 967–978
Jerison, H. J. 1973. Evolution of the brain and intelligence. Academic Press, New York.
Northcutt R. G. 2002. Understanding vertebrate brain evolution. Integrative and Comparative Biology 42: 743–56.
Rowe, T. 1996. Coevolution of the mammalian middle ear and neocortex. Science 273:651-654.
Simpson, G. G. 1927. Mesozoic Mammalia. IX. The brain of Jurassic mammals. American Journal of Science 214:259-268.
Smulders, T. V. 2009a. Darwin 200: special feature on brain evolution. Biology Letters 5: 105–107.
Smulders, T. V. 2009b The relevance of brain evolution for the biomedical sciences. Biology Letters 5, 138–140.
Wang, Y.-Q., Y.-M. Hu, J. Meng, and C.-K. Li. 2001. An ossified Meckel’s cartilage in two Cretaceous mammals and origin of the mammalian middle ear. Science 296: 357—361. (with 80 pages of supplementary data).

Jin Meng

Division of Paleontology, American Museum of Natural History, New York, NY 10024, USA. jmeng@amnh.org

Biography

Jin Meng PhD
Curator and Professor
Division of Paleontology
American Museum of Natural History
Central Park West at 79th St.
New York, NY 10024
Tel: 212-496-3337; Fax: 212-769-5842; email: jmeng@amnh.org

Education
09-1985---06-1991:  Ph.D., Columbia University, affiliated with the American Museum of Natural History (major: vertebrate paleontology; advisor: Dr. Malcolm C. McKenna).
09-1982---10-1984: M.S., the Graduate School of Sciences and Technology, Beijing, and the Institute of Vertebrate Paleontology and Paleoanthropology, Academia Sinica, Beijing (major: vertebrate paleontology).
02-1978---02-1982:  B.S., Beijing University (major: geology).

Position
07-1999---06-2002: Assistant Curator, Division of Paleontology, American Museum of Natural History, New York.
06-2002—present: Associate Curator, Division of Paleontology, American Museum of Natural History, New York.
10-2007---present  Research Associate, Carnegie Museum of Natural History
07-2002---05-2007  Associate Curator, American Museum of Natural History
07-1999---06-2002  Assistant Curator, American Museum of Natural History
01-2000---present  Adjunct faculty, the Biology Department of the City University of New York.
01-2000---present  Adjunct faculty, the Department of Earth and Environmental Sciences of the City University of New York.
01-1996---present  Research Associate, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing.
09-1996---06-1999     Assistant Professor, Department of Biology, University of Massachusetts, Amherst.