sexta-feira, 13 de maio de 2016
Macacos sem cauda
«As with other atavistic structures, human tails are most likely the result of either a somatic mutation, a germline mutation, or an environmental influence that reactivates an underlying developmental pathway which has been retained, if only partially, in the human genome (Dao and Netsky 1984; Hall 1984; Hall 1995). In fact, the genes that control the development of tails in mice and other vertebrates have been identified (the Wnt-3a and Cdx1 genes; Greco et al. 1996; Prinos et al. 2001; Schubert et al. 2001; Shum et al. 1999; Takada et al. 1994). As predicted by common descent from the atavistic evidence, these tail genes have also been discovered in the human genome (Katoh 2002; Roelink et al. 1993). As discussed below in detail, the development of the normal human tail in the early embryo has been investigated extensively, and apoptosis (programmed cell death) plays a significant role in removing the tail of a human embryo after it has formed. It is now known that down-regulation of the Wnt-3a gene induces apoptosis of tail cells during mouse development (Greco et al. 1996; Shum et al. 1999; Takada et al. 1994), and similar effects are observed in humans (Chan et al. 2002). Additionally, researchers have identified a mutant mouse that does not develop a tail, and this phenotype is due to a regulatory mutation that decreases the Wnt-3a gene dosage (Greco et al. 1996; Gruneberg and Wickramaratne 1974; Heston 1951). Thus, current evidence indicates that the genetic cause of tail loss in the evolution of apes was likely a simple regulatory mutation(s) that slightly decreased Wnt-3a gene dosage. Conversely, a mutation or environmental factor that increased dosage of the Wnt-3a gene would reduce apoptosis of the human tail during development and would result in its retention, as an atavism, in a newborn.»
Enquanto que chimpanzés por exemplo não têm cauda, o facto de nós apresentarmos cauda, só afirma que temos ambos uma ancestral comum com cauda, pois a nossa relação filogenética já é evidenciada pela genética, e detecção de retrovírus no genoma, entre outras evidências.
Para mais informações consultar a página web: http://www.talkorigins.org/faqs/comdesc/section2.html#atavisms_ex2
Alternativamente, consultar o e-book em pdf: http://www.talkorigins.org/pdf/comdesc.pdf