Legs evolved only at the end!

Abstract

Talking about legged locomotion often evokes the idea that animals using such devices are perfectly adapted to this kind of motion and should be copied by robotics. The aim of this contribution is to show that the evolution of legs comes late in phylogeny, be it in arthropods or vertebrates. Neural control of legs in vertebrates has to deal with conservative arrangements 'invented' for axial locomotion of metameric organisms. The structure of this paper is to show the importance of axial driven propulsion in vertebrates without legs, with legs and only at the end how limbs move the body in eutherian mammals.

Walking: technology and biology

“…In the following, we shall give some examples from various fields of biology, where walking performance has been investigated, including a very 'biological' design of a walking machine. Fischer & Witte (2006) reinforce some of these ideas stating that the evolution of legs comes late in phylogeny and that 'biologically inspired' technologies might be a better and a more moderate approach than bionics or biomimetics. They continue that on the one side technological structures are always designed anew, but that on the other side biological structures are always the result of a past, permanent and ongoing process, which means 'derived from ancestors'.…”

“…The same idea of a close cooperation of biologists and engineers has been pursued at the University of Karlsruhe (see Dillmann et al 2006) by Professor Dillmann and his group in developing the four-legged machine BISAM, the morphology and behaviour-based control of which follows as near as possible biological findings, especially those of Fischer in Jena (see Fischer & Witte 2006). Considering also some ideas from researchers in the US and Japan, the behaviour-based architecture forms a behaviour coordination network by connecting the inputs and the outputs of the behaviours.…”

Free-standing leaping experiments with a power-autonomous elastic-spined quadruped

“…Because Canid was designed to explore just this transfer of energy between the legs and spinal element relative to their role in promoting efficiency and speed, we are happy that bidirectional operation is available, but we have not yet seen much evidence of its appearance in the preliminary leaping data we present here. Canid's powerful flexible spine is specifically motivated by evolutionary evidence that vertebrate legs first appeared as spring struts primarily actuated by strong, segmented, undulatory trunks responsible for propulsion as animals emerged from the sea [17]. This same hypothesis is explored with greater purity of focus in the recent Kitty design [30], [49], which features a richly parameterized family of cabledriven compliant spine assemblies permitting both sagittal and coronal plane bending and with no other actuation at all.…”

Empirical validation of a spined sagittal-plane quadrupedal model

“…Yet decades of work on legged platforms [1] has thus far largely yielded designs that attach legs to rigid-bodies, despite the abundance of morphological diversity in biology such as tails and spines that contribute to locomotion prowess. In particular, locomotion using a flexible trunk or spine is poorly understood in robotics despite its fundamental role in biological legged locomotion [2]. Throughout this paper we use "spine actuation" and "core actuation" to refer to actuated degrees of freedom proximal to rather than distal from the mass center.…”