Complex brain and optic lobes in an early Cambrian arthropod

摘要

“寒武纪大爆发”指距今5．30亿年前的一段时rn间，当时具有现代特征的动物首次出现在化石rn记录中。寒武纪节肢动物的化石显示了复杂的rn感官，如复眼，但神经系统其他部分通常在形rn成化石之前便腐烂掉了。这篇论文介绍了来自rn中国的一种早期节肢动物的一个保存极为完好rn的脑，它有与现代昆虫和甲壳类非常像的触觉rn神经、视束和视神经。这表明，如果昆虫是从rn非常简单的生物如“叶足(腮足)虾”演化来rn的，那么现代叶足动物神经系统的复杂性与远rn古时代相比应当是大大降低了。%The nervous system provides a fundamental source of data for understanding the evolutionary relationships between major arthropod groups. Fossil arthropods rarely preserve neural tissue. As a result, inferring sensory and motor attributes of Cambrian taxa has been limited to interpreting external features, such as compound eyes or sensilla decorating appendages, and early-diverging arthropods have scarcely been analysed in the context of nervous system evolution. Here we report exceptional preservation of the brain and optic lobes of a stem-group arthropod from 520 million years ago (Myr ago), Fuxianhuia protensa, exhibiting the most compelling neu-roanatomy known from the Cambrian. The protocerebrum of Fuxianhuia is supplied by optic lobes evidencing traces of three nested optic centres serving forward-viewing eyes. Nerves from uni-ramous antennae define the deutocerebrum, and a stout pair of more caudal nerves indicates a contiguous tritocerebral component. Fuxianhuia shares a tripartite pre-stomodeal brain and nested optic neuropils with extant Malacostraca and Insecta, demonstrating that these characters were present in some of the earliest derived arthropods. The brain of Fuxianhuia impacts molecular analyses that advocate either a branchiopod-like ancestor of Hexapoda or remipedes and possibly cephalocarids as sister groups of Hexapoda. Resolving arguments about whether the simple brain of a bran-chiopod approximates an ancestral insect brain or whether it is the result of secondary simplification has until now been hindered by lack of fossil evidence. The complex brain of Fuxianhuia accords with cladistic analyses on the basis of neural characters, suggesting that Branchiopoda derive from a malacostracan-like ancestor but underwent evolutionary reduction and character reversal of brain centres that are common to hexapods and malacostracans2. The early origin of sophisticated brains provides a probable driver for versatile visual behaviours, a view that accords with compound eyes from the early Cambrian that were, in size and resolution, equal to those of modern insects and malacostracans.