Introduction

We humans have always been intrigued by the astonishing cases of structural or functional similarities between body parts found in the animal kingdom (e.g., wings of insects and birds, eyes of squids and vertebrates, and so on). This issue was deeply investigated by the nineteenth-century naturalists and anatomists, who gained access to a large variety of new species, thanks to the European exploratory voyages to new worlds, favoring their 'intellectual voyage' in search of the archetype. Owen was one such anatomist who realized that not all similarities are equal, coining the term 'homologue' to define ''the same organ in different animals under every variety of form and function'' (Owen, 1843; A History of Ideas in Evolutionary Neuroscience; see also Mayr, 1982; Raff, 1996; Panchen 1999; Butler and Saidel, 2000; Puelles and Medina, 2002; Wake, 2003). He also coined the term 'analogue' to define a part or organ in one animal that has the same function as another part or organ in a different animal (Owen, 1843; Wake, 2003). Soon after the introduction of these terms, evolution came into the scene to explain why similarities exist (Darwin, 1859), and evolutionary biologists used the term 'homology', under a new perspective. For evolutionary biologists, two structures are homologous if they can be traced back to a common ancestor (i.e., homology is similarity due to inheritance from common ancestor) (Striedter and Northcutt, 1991; Raff, 1996; Gilbert and Burian, 2003; Wake, 2003). In this view, the existence of homologues is the consequence of evolution, and the study of homologies is, in essence, the study of evolution. In addition to homology, another key concept for evolutionary biologists is 'homoplasy', a term coined by Lankester to define similarity not linked to common ancestor, but due to convergent or parallel evolution (Lankester, 1870; Hall, 1999; Panchen, 1999; Wake, 2003). To discern between homology and homoplasy is not an easy task since it is difficult to know what the prevalent characters are that should be chosen for comparison. Moreover, many problems arise when trying to consider intriguing cases such as similarities not linked to common ancestor but being produced by a similar developmental pathway (i.e., sharing a common generative pathway), or similarities linked to common ancestor but produced by different developmental mechanisms (Striedter, 1998; Butler and Saidel, 2000; Puelles and Medina, 2002; see A History of Ideas in Evolutionary Neuroscience). One very useful way to approach the study of homologies is by combining developmental and evolutionary views (Gould, 1992; Raff, 1996; Gilbert et al., 1996; Striedter, 1998; Gilbert and Burian, 2003), which makes sense since evolution occurs as a consequence of changes in developmental mechanisms that produce phenotypic variations, which are then exposed to natural selection (de Beer, 1951; Waddington, 1966; Gilbert et al., 1996; Raff, 1996). This combined 'evo-devo' approach has provided unequivocal evidence for the modular nature of embryos, and for the existence of discrete fields, the morphogenetic fields, which represent major ontogenetic units (modules) able to change independently, producing variation and diversity in evolution (Raff, 1996; Gilbert et al., 1996; Gilbert and Burian, 2003; Gass and Bolker, 2003). This evidence has been extremely important for rescuing the concept of 'field homology', first introduced by Smith (1967) to define correspondence due to derivation from a common embryonic anlage, and used later by comparative neurobiologists to homologize brain regions assumed to develop from homologous precursors (Northcutt, 1981; Butler, 1994a, 1994b; Striedter, 1997). Based on the importance of morphogenetic fields as major ontogenetic and phylogenetic units, the concept of field homology has recently been reformulated under the modern perspective of evolutionary developmental biology, to be used in a dynamic rather than static sense, as one of the natural comparison characters for evolutionary studies in general and, in particular, for studies of brain evolution (Puelles and Medina, 2002). Below I briefly summarize some ideas related to this concept.

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