DEVELOPMENTAL PROCESSES, DEVELOPMENTAL SEQUENCES AND EARLY VERTEBRATE PHYLOGENY

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Abstract
Summary: (1) We have put forth the position that evolutionary sequences can be deduced by an analysis of fundamental developmental sequences. Such sequences are highly conserved within a group and ‘contain steps which are necessary to achieve a developmental fate’. The premise of our work then, is that such fundamental sequences do not arise de novo time and time again but can be traced back through their evolutionary history in organisms which contain portions of the sequence.(2) These highly conserved developmental sequences are in fact developmental constraints to evolution in as much as natural selection has not been able to discard them, but rather has utilized them in achieving evolutionary change.(3) We have demonstrated the ability to use developmental data by producing an evolutionary sequence for the origin of the vertebrates using the processes of neuralization and cephalization, the latter due primarily to the influences of the neural crest and epidermal placodes. The evolutionary sequence created, while not novel in structure, is distinct in that it was created solely by following a developmental sequence that is highly conserved among the vertebrates. The sequence is:(a) Chordamesoderm differentiates from the surrounding mesoderm and induces an overlying neural tube.(b) Through the influence of neuralizing morphogens, the neural tube differentiates into anterior (fore‐, mid‐ and hindbrain) and posterior (spinal cord) parts. Cephalization has begun.(c) Cephalization proceeds via the development of two new populations of embryonic cells, the neural crest, a derivative of the neural epithelium and the epidermal placodes, derivatives of the ectoderm immediately adjacent to the neural tube. These two populations contribute significantly to the subsequent development of the vertebrate head including the skeleton, connective tissues, cranial nerve and sensory organs.Sequence (a) occurs in the most primitive protochordates and is one of the differences between the chordates and deuterostome invertebrates. Sequence (b) occurred next leading to a protochordate with a differentiated central nervous system, but lacking most vertebrate head structures. Sequence (c) signalled the beginning of the true vertebrates or branchiates (after the branchial arches which all vertebrates' share) since the production of a neurocranium, viscerocranium, cephalic armour, teeth and cranial peripheral ganglia was only possible with the acquisition of this developmental step.(4) Current investigations into the cellular, molecular and biochemical basis of developmental sequences will allow biologists to test for the integrity of these developmental sequences and to test for their presence in the ontogenies of any species in question. These investigations will ascertain the validity of determining an evolutionary sequence based on a particular developmental process(es) as outlined in the present paper.