The first Metazoa animals possessed a modular structure, consisting of several identical individuals combined into a colony. The colony appeared due to gemmation, where sponges grow out of the body of another sponge, to form a colony of perfectly identical creatures. A sponge colony is motionless. If the environment changes, the colony cannot move but can change shape. This happens as follows: some members of the colony are born, others - atrophy, or, as the researchers say, undergo involution.
A signal to involution for some members is formation of other members. The members of a growing colony use substances released after the old members atrophy as building materials. Since all modules of the colony are genetically identical, the death of some part of them does not damage the gene pool. Therefore, this ancient extinction mechanism served exclusively for changing the colony shape and did not promote evolution of modular organisms.
In the course of evolution, animals reproducing in an asexual way started to separate completely from each other. A lot of Polychaeta worms can reproduce by division or even by gemmation, but each derived organism already lives separately. However, in such way of reproduction, gemmation of descendants causes involution of parental tissues as before.
The difference is that the parental organism dies in this case, so, in A.V. Makrushin’s opinion, this can be already considered ageing. However, such ageing does not impact the evolution yet, because the genotype of the parents that died of old age continues to exist in their offsprings. However the Metazoa animals continued to evolve and finally did away with gemmation. Each of their offsprings, with the exception of monozygotic twins, possessed a unique genotype, and the loss of an individual was an irreplaceable genetic loss. And the mechanism of gerontal involution of tissues, which was preserved by these advanced creatures, resulted now not only in the death of a single individual, but in changing the gene pool of an entire species. Thus ageing began to influence the species evolution.
All Metazoa animals are descendants of modular organisms, from which they inherited ability to gerontal tissue involution. This is a universal and very ancient mechanism of ageing. Probably, it originated several times in the course of evolution of different groups of modular multicellular animals.
A.V. Makrushin also gives other examples where, from his point of view, ageing happens according in line with other mechanisms. For example, some hydra species die a natural death, having completed reproduction, but some adult insects die of hunger as their oral organs are underdeveloped. Their life span is determined by the fat reserve accumulated during their larval existance.
However, the primary ageing mechanism of the multicellular is gerontal tissue atrophy. Now, this mechanism is a key part of evolution, including human development, which means we have to grow old.
Nadezda Markina | alfa
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