Why T rex had shorter hands so as Kangaroos?

This theory tells about the correlation between metabolic gradient and evolution. I want to say that I am inspired by the metabolic gradient theory of C. M. Child, which simply states that metabolic rate is higher in the head region and gradually decreases towards the tail region. Above theory motivates my thinking and I apply this theory to the evolutionary processes and found that this theory is applied with some exceptions. In simple words, this theory states that evolutionary agents work faster in the head region and gradually become slower in the tail region because of metabolic gradient. This may be because of the nervous system which is well developed in the head region and gradually decreases towards the tail region. Nervous system develops earlier than most body parts so nerve growth factors secreted by them may be responsible for that. That’s why this theory applied does not apply to organs which developed earlier than the nervous system.

What about the animals which does not have nervous system they would be asymmetrical. This also explains the symmetry of an organism. Body’s shape is relative to the nervous system. One other reason is to have more mitochondria in the anterior part of the body which tends to utilize more oxygen in the cell and this oxygen play vital role in cell metabolism. When we see animal’s extremities such as limbs, etc, we found that forelimbs are longer or shorter than the hind limbs. This asymmetry is the main question of my theory that why forelimbs are modified in different organisms according to divergent evolution but hind limbs are not.  This may be due to different metabolic rates in these regions which decide where the evolutionary rate will be faster or slower.

Evolutionary forces such as mutation works at higher rate in the anterior parts as compared to posterior parts of the body. By this we can predict the past and future size of the organism. For example Giraffe has a longer forelimbs than hind limbs means we can predict that species of Giraffe is gradually increasing in size during evolution because forelimbs will increase first due to faster evolutionary process in the anterior part of the body. Other examples are Kangaroo, Tyrannosaurus rex and Hyenas whose forelimbs are shorter than hind limbs means they are gradually decreasing in size during evolution. We can calculate this difference in the hind limbs and forelimbs by humerofemoral index.

Humerofemoral index = Length of humerus/ Length of femur*100                           

If humerofemoral index is less than 100, then animals tend to decrease in size over evolutionary time and if more than 100, then it tends to increase in size over evolutionary time. In anurans hind limbs are longer than forelimbs means these are gradually decreasing in their size or they will be shorter in size in the future. If we look at the anterior half of animals having same genus, we found more variations, but if we observe the posterior half we see the minor variations. If we see lining in the zebra’s body, it is more in the head, but is less in abdomen this also due to metabolic gradient.  This theory can also be tested in dorsal and ventral side of the body or from internal to external side of the body. The hepatic portal system evolves earlier than the renal portal system because the former lies in the anterior part of the body.

Another evidence of this theory is that if we see arthropod legs we see that prothoracic legs are more modified than the metathoracic legs due to metabolic gradient by this we can predict how they have evolved from simpler forms which is still present in hinder body parts.  One other evidence is the presence of vestigial hind limbs in pythons and boas and forelimbs are completely extinct, it shows that evolution plays faster and earlier role in the anterior part of the body than posterior part of the body.  Mole Lizard is seems to be an exception to this theory.