Endocasts: A Superficial Look At Ancient Brains

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PhrenologyPixBack in the 19th century, a very popular neurological research program developed: phrenology. Its proponents posited that the brain is a collection of many organs, each organ leading to a specific personality trait. The diagram above is a phrenological one, and the phrenologist would feel up and measure the bumps on a person’s skulls. The logic was simple: if a person has a bump, then the brain region beneath the skull must be larger, and therefore whatever that region represented would be highly-manifested in that person. So, according to the chart above, the skull around my ear would be a great cavity because I’m not active, destructive, or very hungry. But I would have bumps around my eye because of my serial multilinguality and my obsession with order.

Needless to say, this is all pseudoscientific poppycock. Most importantly, the brain does not work that way – rarely are personality traits mappable to individual sections. The outside of the skull can be modified by muscular attachments (e.g. the jaw muscles) and by mechanical damage, but it does not reflect the state of the brain underneath it.

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However, the brain can influence the inside of a skull. You all know what the outside of a human brain looks like: it’s a dense maze of grey matter grooves and ridges (sulci and gyri, respectively). This is the cerebral cortex.

If you look at the inside of a human braincase, you will see that the cerebral cortex’s torturous pattern is etched onto it. The reason why this happens is because the skull is made of bone, and bone is a living tissue that responds to local mechanical stimuli, such as the brain pushing up into it. The skull is too thick for these effects to be seen on its outside, but they are clear enough on the inside that they are even fossilised.

Vertebrate palaeontologists can take a fossil skull and fill it with latex rubber to make a mould. This is called an endocast, short for endocranial cast. Nowadays not even that is needed, you can just shove a skull in a CT scanner to get digital reconstructions. Very rarely, an endocast can be formed naturally when the corpse’s skull gets filled with mud or other fine sediment. Regardless of how you get an endocast, because the animal to whom it belonged had a brain in its skull, the mould might show the pattern of the outside of the brain, and palaeontologists can then use this to study palaeoneurology, a field founded by Ralph Holloway, a household name for anyone even remotely interested in the evolution of human brains.

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Human endocast. Source: Holloway (2010)

The type and amount of information that can be gleaned is limited, even if one gets a perfect endocast. Compare the endocast up there with the brain picture a few paragraphs ago – you don’t really get that much of the cerebral cortex preserved, and nothing at all from the inside of the brain. Nonetheless, endocasts provide invaluable glimpses into the evolution and development of brains, and thus indirectly into the behavioural biology of animals, and they remain the only direct evidence for how ancient brains looked like. The foundational logic behind palaeoneurology is that the amount fo neural tissue present ina certain area correlates positively with the importance of that area’s fuinction. So if an animal’s endocast shows a very large visual lobe, then vision must have been highly-sophisticated in that animal.

The study of endocasts began with fossil humans, and so endocasts are critical in palaeoanthropology. One of the most important fossils ever discovered, the Taung Child, is notable both for its facial features and for the impeccably preserved endocast, painstakingly recovered from rock by Raymond Dart using his wife’s knitting needles in 1924. Topics ranging as varied as the evolution of language and the taxonomic status of Homo floresiensis are informed by endocast studies. For example, H. floresiensis, the “hobbit” human found on the Flores Island, had long been at the center of a controversy about whether it’s a true insular dwarf species derived from Homo erectus, or whether it’s a regular H. erectus suffering from microcephaly. Endocasts were used in the elucidation of its true status; see the exchange between Martin et al. (2006) and Falk et al. (2006).

As for the evolution of human language, endocasts are valuable in revealing the taxonomic distribution of Broca’s area, a region of the cerebral cortex involved in facial coordnation and in language processing. It appears to have been rudimentarily present in all Homo species, and some have even argued for it being found in australopithecines. In either case, what that tells us is that the potential capacity for language has long been present in humans, not something that appeared out of thin air in Homo sapiens.

For scientists interested in mammals, it has been known for a long time that since the mammalian brain pretty much fills up all the braincase, endocasts provide a very faithful representation of the relative sizes of different brain regions and can thus deliver otherwise unknowable information on the evolution of brains. For example, investigations of the endocasts of many early primates reveal a small brain with relatively large olfactory bulbs, hinting at early primates being highly-dependent on their sense of smell, with the visual centers becoming more prominent as primates evolved further (Silcox et al., 2009, 2010).

Mammal brains are probably the most exciting to study due to the unique presence of the isocortex, the result of an encephalisation process. The first sign of an isocortex can be found in the endocasts of early mammals (Luo et al., 2001), but not in the endocasts of theraspid ancestors of mammals (Kemp, 1982).

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Endocast of Archaeopteryx. Source: Alonso et al. (2004)

Of course, endocasts are found in all craniates, so they can be useful for any group with a skull. Dinosaur palaeontologists are fond of using them to find out whatever they can about their animals’s brains. The most popualr dinosaur endocast is Stegosaurus‘s laughably ridiculous 60cm³ brain. But there are more serious dinosaur endocast facts that can be thrown around. For example, Rogers (1999) made a virtual endocast of an allosaur and found its brain was arranged more like a crocodile’s than a bird’s, a similarity since corroborated by several other dinosaur endocasts – although as you move into the close avian relatives, like the Archaeopteryx whose endocast is imaged above, the brain becomes bird-like (Alonso et al., 2004). Carnivorous dinosaurs also tend to have relatively large olfactory bulbs, which can be useful for both detecting prey (predators) and decaying carcasses (scavengers).

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Reconstructed brain of Shuyu zhejiangensis. Source: Gai et al. (2011)

The picture above shows the virtual endocast of Shuyu zhejiangensis, an early galeaspid fish from the Silurian of China. In other words, you are looking at what the brain of a 430 million year old fish looked like. If that’s not an awesome testament to the prowess of modern palaeontology, I don’t know what is.

As I said before, endocasts are not perfect. They are not casts of the brain, but merely of the impression made by the outermost layer of the brain on the inside of the skull. Endocasts do not hold a candle to studies of actual brains. However, what I have tried to show in this post is that even with a fossil record subject to unavoidable, uncountable losses, there are ways to gain an otherwise unattainable direct look into the evolution of an unfossilisable organ like the brain.

References:

Alonso PD, Milner AC, Ketcham RA, Cookson MJ & Rowe TB. 2004. The avian nature of the brain and inner ear of Archaeopteryx. Nature 430, 666-669.

Falk D, Hildebolt C, Smith K, Morwood MJ, Sutikna T, Jatmiko, Saptomo EW, Bundsen B & Prior F. 2006. Response to Comment on “The Brain of LB1, Homo floresiensis. Science 312, 999.

Gai Z, Donoghue PCJ, Zhu M, Janvier P & Stampanoni M. 2011. Fossil jawless fish from China foreshadows early jawed vertebrate anatomy. Nature 476, 324-327.

Holloway RL. 2010. Human Brain Endocasts, Taung, and the LB1 Hobbit Brain. In: Broadfield D, Yuan M, Schick K & Toth N (eds.). The Human Brain Evolving: Paleoneurological Studies in Honor of Ralph L. Holloway.

Kemp TS. 1982. Mammal-Like Reptiles and the Origin of Mammals.

Luo Z-X, Crompton AW & Sun A-L. 2001. A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics. Science 292, 1535-1540.

Martin RD, MacLarnon AM, Phillips JL, Dussubleux L, Williams PR & Dobyns WB. 2006. Comment on “The Brain of LB1, Homo floresiensis. Science 312, 999.

Rogers SW. 1999. Allosaurus, crocodiles, and birds: Evolutionary clues from spiral computed tomography of an endocast. The Anatomical Record 257, 162-173.

Silcox MT, Dalmyn CK & Bloch JI. 2009. Virtual endocast of Ignacius graybullianus (Paromomyidae, Primates) and brain evolution in early primates. PNAS 106, 10987-10992.

Silco MT, Benham AE & Bloch JI. 2010. Endocasts of Microsyops (Microsyopidae, Primates) and the evolution of the brain in primitive primates. Journal of Human Evolution 58, 505-521.