Different animals have vastly different leg arrangements. Animals lower on the tree of life have legs with an overall form which is analogous to higher animals, but the final shapes, leg attachment schemes, and means of locomotion are very different.

<| Rotated Leg Attachments

Lower Vertebrates. Near the bottom end of the vertebrate evolutionary tree is the salamander, whose legs project nearly straight out from the torso, as shown on the left. Salamanders are amphibians, which are born in the water and go through a larval stage with gills and no legs. On transforming into adults, they lose the gills, develop legs, and live on the land. When the salamander walks, its vertebral column bends back and forth, and leg movement is closely linked to this bending. This spinal action is very similar to that of a fish swimming in water, which suggests a strong tie to the salamander's early swimming instincts while in larval form. Leg Evolution. Higher vertebrates, in contrast, show little "sideways" bending of the spine during walking. The legs act much more independently than in the salamander, as a result of a wholesale re-orientation of the leg attachments to the torso, as shown on the right. Some of the major adaptations are as follows: the legs are rotated towards each other at shoulder and hip, and relocated closely under the body, which allows them to more easily hold the body in an upright posture. the knees/elbows and ankle/wrists are bent, so as to form more of a vertical support column; limb movements are now more "accordion-like" than "lever-like", which gives greater power, balance, and spring to the step. the lower part of the front legs are twisted so that the foot pads [palms] face towards the ground. The last point is quite interesting. The lower leg twist is necessary because the front leg overall rotates so as to point "backwards", in which case an "untwisted" leg would have the foot pads pointing to the sky - clearly not very convenient. So why did the front legs not rotate to point forwards, like the back legs? As described in more detail elsewhere, the overall scheme is that the re-aligned legs front-back operate in a mirror-image fashion - allowing large leg extensions and retractions during walking and running, while at the same time keeping the COG of the body relatively unchanged within the torso. This is obviously more advantageous than an arrangement employing non-mirror-image leg movements, which would force the animal to compensate for large COG shifts during each and every movement. See the drawings of the horse and cheetah below.

<| Walk Like a Dino

Mammals did not actually invent the rotated-beneath-the-body leg arrangement. In fact, going back in far time, many species of dinosaur also had their legs located so. How else to support up to 80 tonnes without having to hold the equivalent of deep knee bends all day? For more on dino walking: [1] [2] [3] [4]. The drawing at the left shows three postures employed by both ancient and modern reptiles. Upper is the sprawling stance, used by early reptiles and today's lizards, turtles, and salamanders. Middle is termed the self-improved stance, used by early and modern crocodilians. Bottom is the fully-improved ["upright"] stance, used by many dinosaurs and later by mammals. The last also led to bipedal dinosaurs, like T-rex. For more about stances: [1] [2] [3]. For incredible pictures of crocodiles galloping: [1] [2] Lycaenops - an ancient mammal-like reptile 250 million years ago, and before the dinosaurs, there was a group of ancient mammal-like reptiles called therapsids [cf, synapsids], some the size of large dogs. They were present in great numbers, and their fossilized skeletons are almost indistinquishable from modern mammals - among other features, they also had a "rotated" leg posture and upright stance. More about synapsids: [1] [2] [3] [4] [5] [6] [7]. All in all, these subtle changes are what make locomotion in vertebrates so successful. See also, origin of mammals, and Peter Dilworth's incredible robotic Troodon [1] [2] [3].

<| Cantilevered Walkers

Arthropods. Moving further back in evolutionary time from the salamander, we find the phylum of Arthropods - jointed-legged, segmented animals. These include insects, bugs, beetles, crabs, spiders, mites, centipedes, etc. Beetles, as shown on the left, and a huge number of other insects like ants, flies, mosquitos, bees, roaches, and various other bugs, have 6 legs. Arachnids [spiders] have 8 legs. Some, like millipedes and centipedes, have many. A typical arthropod leg form is shown on the right. It has several segments, which are analogous in form to vetertebrates, including femur, tibia, and multiple [4-5] tarsal segments. The coxa provides an extra degree-of-freedom to the leg, similar to that provided by the shoulder in mammals. Some legs have additional segments. Of special interest is that, like in the salamander, the legs project out from the body, as opposed to being located underneath the body as in mammals. Because of this, the legs are oriented and move in much different ways than in mammals, as shown by the beetle on the left. The upper leg segments generally point upwards and the lower segments downwards. The tarsals scrape the ground. Looking over many pictures of 6-legged arthropods shows that the 4 rear legs typically point backwards while the 2 front legs point forwards. Two Theories. An obvious question is, why are there no beetles [or other arthropods] with only 4 legs? Pure conjecture --> 4 legs are in general too weak and too few to hold up and move the body efficiently, when they extend outwards away from the torso, as opposed to being oriented directly beneath the torso. All the 4-legged beetles were squashed by dinosaurs 65,000,000 years ago, because they couldn't get out of the road fast enough. So much for brilliant theories ... it may simply be that arthropods have 6 [or more] legs so they can use the middle and rear to stand and move on while the front are used for probing, grasping, fighting, and eating. In fact, some arthropods are uniquely adapted to this - eg, the praying mantis has 2 enormous, highly-specialized front legs plus 4 regular-looking ones under the thorax. Better to have 4 feet for running plus 2 left over for simultaneous fighting, than to have only 4 total and to have to stop running in order to use 2 for defense. It was probably not the dinosaurs who eliminated [possible] early classes of 4-legged beetles, but rather their better-designed 6-legged enemies. Tripod Gait. A common 6-legged gait is tripod-to-tripod - front-back legs on one side touching the ground together with the middle leg on the other side to form one tripod, alternating with the opposite arrangement. Conceptually, this type of gait is both statically and dynamically stable. An arthropod resting on one tripod will not fall over. Compare this to a quadruped standing on one diagonal, or a biped standing on a single leg - both of these are dynamically, but not statically, stable. Click here for tripod gait in action. And interestingly - someone has come up with "whegged" robots that use 4- or 6-wheels, each with 3-legs, and which do classic tripod and diagonal gaits - see RHex and whegs. See also, arthropod leg modeling.

<| Empowered Locomotion

Most arthropods have rather wimpy looking legs, compared to higher vertebrates, and their walks seem more akin to skittering than to powerful thrusting. When the rubber [foot pad] meets the road, a crab or beetle is clearly no match for the power and efficiency of a horse or cheetah. The re-alignment of the legs to underneath the body, described above, was clearly the key feature which allowed eventual development of larger, more powerful body types. In comparison, no insects are very large or overly powerful, although a few, like the grasshopper, have rear legs especially adapted for powerful jumping. The largest vertebrates with side-projecting legs are alligators and crocodiles, and while powerful, these animals are slow and cumbersome when out of the water - although some may be deadly over short distances; see [1] [2]. On the other hand, roaches are reputed to be the fastest insects around, having been clocked up to 3.4 MPH [1.5 m/s]. They use 6-legged gaits at low speeds, but are said to employ both 4-legged and ultimately 2-legged gaits, when running at high speeds. See the work of R.J. Full. Although the roach is undoubtedly unaware of it, its ontological adaptations appear to pre-curse later changes in locomotive phylogeny.

<| Summary

Lower animals, as far back as arthropods, have multi-segmented legs analogous to mammals, but vastly different attachment geometries and gaits. Lower in the chain of life, legs project outwards from the body and require more of a levering action to hold the body up. Higher up in the chain of life, leg attachments have re-oriented so the legs are located vertically beneath the body; this allows the animal to stand upright with little expenditure of energy, and also greatly increases its power and stride. The legs of higher animals have a bent geometry, with corresponding front-back leg joints bending in a mirror-image fashion with respect to each other. Multi-legged animals employ different gaits as a means to co-ordinate the many legs during movements; the tripod-to-tripod gait is common for 6-legged creatures.

<| References