Although it’s not something we think about that often, we probably all know what lizards look like, and that they certainly don’t run the same way we do.
That’s because they still retain many of the primitive features of our last common ancestor including scaly skin and egg-laying. The most important feature we’re concerned with here though is their limb anatomy. When animals develop as an embryo each muscle develops from one of 2 sets of cells, either dorsal (back) or ventral (front), and that is reflected in their nerve supply in later life. Ventral muscles flex joints while dorsal muscles extend them.
This is all well and good in a lizard because the muscles stay where they are for its entire life – we say the dorsal and ventral compartments are well-defined (see fig. 1).
As we can see in fig. 1 a lizards limbs stick out to the side of their body, which means that they can’t reach forward in the same way that we can with our hips or shoulders. Their extensor muscles raise their foot off the ground and their flexors bring it back to the ground. To make forward progress they have to undulate their bodies as in fig. 2 below.
As ever, the situation in humans is not as clear-cut. In fig. 3 below we can see that some joint motions aren’t quite right.
Our hamstrings are located at the back of our thigh (are dorsally placed) but they act to flex our knee. This means they must be developmentally ventral. Similarly, our quadriceps are at the front of our thigh (are ventrally placed) but their job is to extend our knee which means they must be developmentally dorsal.
You may also have noticed that because these muscles are in the ‘wrong’ place, they also have opposite effects on our feet than those of lizards. In humans, our flexor (hamstring) raises the foot from the ground, while the extensor (quadriceps) lowers it again.
This all happens because we start our embryological development in a very similar manner to a lizard in its egg, but quite early on (about week 7) the protruding paddles which will become our limbs rotate in opposite directions (Barbaro-Brown, 2008). To demonstrate this, here’s something you can try yourself.
DO try this at home:
First, sit with your arms out to the side with palms facing forward. Bend your elbows, keeping your palms facing forward and thumbs pointing up. Your arm should look like the lizard’s leg in figure 1c. Next, spread your knees as wide as possible and try to bring your feet up so your soles are facing forward too. Your leg should look like the lizard’s leg in figure 1c, too, with your quadriceps now facing backward and your hamstrings forward.
Congratulations – you’ve just demonstrated that the quads are developmentally dorsal and the hammies are developmentally ventral!
To mimic the rotation that occurs at about week 7 of development, bring your legs back to a normal sitting position – your hind limbs have just rotated medially (big toes have been brought toward the midline of your body).
This has brought your quads and hamstrings back to their modern human positions
Now, without moving your elbows or wrists, let your shoulder relax so your upper arm is against your side. Your elbow should be bent so your forearm is pointing forward, and you should have your palm facing front with fingers pointing at the ground and thumbs out to the side – your upper limbs have just rotated laterally (thumbs are now pointing away from your midline).
Obviously this isn’t how our arms normally look and it could get uncomfortable after a while so now you can straighten your wrists leaving your palms facing up. But this still isn’t how we or almost any other mammal hold our upper limbs, so turn your hand over so it’s palm down and you’ve carried out the full set of rotations that distinguish mammals from lizards! And because you’re a primate, you can go and reward yourself with a tasty treat of your choice, using your opposable thumbs to hold it.
My snack of choice? A cheese and Marmite toastie for lunch.
Barbaro-Brown J, 2008. Embryological development of the lower limb. Durham, UK: Judith Brown CPD. Available from: http://www.judithbrowncpd.co.uk. Accessed: 11/01/12. Freely available.
Stern JT, 2003. Essentials of Gross Anatomy: F. A Davis Co. Available from: www.anat.stonybrook.edu. Accessed: 20:21pm, 12/01/12. Freely available.
The MIT Press, 2010. Image available from mitpdev.mit.edu. Accessed 12:12pm, 26/01/14. Freely available.
Kapit W, and Elson LM, 2014. The Anatomy Coloring Book. Pearson, San Francisco.