Jackson School of Geosciences, University of Texas, C1100, Austin, TX 78712, USA.
Science. 2011 May 20;332(6032):955-7. doi: 10.1126/science.1203117.
Many hypotheses have been postulated regarding the early evolution of the mammalian brain. Here, x-ray tomography of the Early Jurassic mammaliaforms Morganucodon and Hadrocodium sheds light on this history. We found that relative brain size expanded to mammalian levels, with enlarged olfactory bulbs, neocortex, olfactory (pyriform) cortex, and cerebellum, in two evolutionary pulses. The initial pulse was probably driven by increased resolution in olfaction and improvements in tactile sensitivity (from body hair) and neuromuscular coordination. A second pulse of olfactory enhancement then enlarged the brain to mammalian levels. The origin of crown Mammalia saw a third pulse of olfactory enhancement, with ossified ethmoid turbinals supporting an expansive olfactory epithelium in the nasal cavity, allowing full expression of a huge odorant receptor genome.
许多假说被提出来解释哺乳动物大脑的早期进化。在这里，通过对早侏罗世哺乳动物摩根齿兽和哈德利齿兽的 X 射线断层扫描，我们对这段历史有了新的认识。我们发现，相对脑容量扩大到了哺乳动物的水平，嗅球、新大脑皮层、嗅觉（梨状）皮层和小脑都有增大，这是通过两个进化脉冲实现的。最初的脉冲可能是由嗅觉分辨率的提高以及触觉灵敏度（来自体毛）和神经肌肉协调性的提高所驱动的。第二次嗅觉增强脉冲使大脑进一步扩大到了哺乳动物的水平。在冠哺乳动物的起源中，嗅球经历了第三次增强，鼻甲骨骨化的鼻甲支持鼻腔内扩张的嗅上皮，从而使庞大的气味受体基因组得以充分表达。
