TIT ORGANS of a body are a spatial division of labor, created by different genes activated in different cells. The same process serves to give individual lives a division of labor over time. Complex algae, animals, fungi, and plants all have predictable life cycles that separate three fundamental aspects of development – the creation of an autonomous individual, growth, and reproduction – and execute them sequentially.
In some creatures, including humans, the transition from one phase to another has obvious continuity. Fertilized eggs turn into fetuses, which grow into babies, which turn into two different kinds of adults, who, between themselves, can then produce new fertilized eggs. In other animals, things are more clearly rhythmic. The embryo that grows inside a butterfly egg transforms into a series of leaf-nibbling and molting caterpillars called stages. The last of these cocoons transforms into a nymph before emerging, winged and fluttering, like an imago with a whole new set of organs. Plants have two distinct life cycles, which alternate from generation to generation, although this is rarely evident to human observers.
Much of the complexity here relates to gender. The fission of a bacterium; budding of a yeast cell; the vegetative growth of a tree pushing offspring: each of them gives offspring genetically identical to the ancestor and to each other. Sex is clearly the start of something new: a new individual with a new genetic pattern and a selfish Darwinian imperative that can put him at odds even with his siblings. Asexual individuals often bond to larger structures – bacterial biofilms, coral heads or aspen forests, for example. Sexual individuals hardly ever do this.
In sexual reproduction, each parent contributes only half of the genome. In asexual reproduction, the entire genome can be transmitted. In terms of explaining biology’s “selfish gene”, a strategy that passes all genes on seems inherently more appealing than a strategy in which half of them are left behind. Sex must therefore provide benefits for which it is worth throwing away half a genome.
The current belief is that these come from the mixture of genes. By producing genetically new individuals, sex poses a problem for parasites and pathogens and provides flexibility under changing circumstances. These advantages compensate for its ineffectiveness. Caenorhabditis elegans, a nematode worm highly regarded in biology labs, reproduces asexually in benign environments but begins to males when the going gets tough, in order to mix things up a bit. That said, mysteries remain – for example, the bdelloid rotifers, which appear to have been exclusively asexual for 25 million years.
Once fertilized, an animal egg develops into an embryo, or something equivalent to one. The role of an embryo is to prepare the ground for further development. It produces what in Silicon Valley you might call a minimum viable product. When a human embryo is born as a baby, it already contains almost all of the organs that individual will ever have.
This first occurs by the repeated division of the initial fertilized egg into numerous cells that have the potential to become any part of the body. Then, around the 16th day of development, the embryo folds into itself in a process called gastrulation. This sees the plane of the body begin to take on physical form, defining the head and tail (for human embryos, indeed, have tails), left and right, inside and out.
After gastrulation, more and more cell lines see their future possibilities reduced, as molecular switches turn off some genes and promote the activity of others. Each cell line is thus guided along a path that leads to its specialization as part of an organ. Some latecomers, called stem cells, abandon this trip. Their role is to generate, throughout the life of an individual, replacements of dead cells. But many specialized cells, especially in muscles and the nervous system, last a lifetime.
Other species have similar stories to tell, but not the same. For example, a butterfly embryo not only develops the organs needed to become a caterpillar, but also starter packs, called imaginal discs, for the organs that will be needed in adulthood.
For most plants, things are more complex because there are two radically different types of bodies. It is still a division of labor, in which mating and dispersal have been separated.
Cells in gametophytes, the body type that mate, have a single complement of chromosomes, a condition known as a “haploid,” which is also seen in the eggs and semen of animals. It is the process of creating haploid cells that sees half of each parent’s genome destroyed during sexual reproduction. Unlike eggs and sperm, however, these haploid cells can grow and differentiate, creating the gametophyte body.
Once it has grown sufficiently, a gametophyte will produce eggs and sperm, which will meet and mate, pooling their chromosomes to create “diploid” individuals that will develop into a body type called a sporophyte. The sporophytes produce haploid spores, which they often seek to distribute as widely as possible, a valuable strategy for a stationary creature.
In mosses, the larger of the two forms is the gametophyte. In ferns, it is the sporophyte, although the gametophyte is still visible to the naked eye. In flowering plants, things went even further, with the gametophyte stage being mainly taken up by the sporophyte. The gametophytes of an oak, for example, are microscopic additives to the body of the sporophyte: the pollen grains born by its male catkins and the embryo sacs of its female flowers.
In flowering plants like oaks, the dispersal of offspring is achieved not by haploid spores, but rather by fertilized embryo sacs creating seeds containing embryos, which in oaks are called acorns. The embryo in an acorn lacks the precursors of many adult organs. The leaves are grown later, as needed, from stem cells called meristems. But it is equipped with a nascent root and stem, and also has two food storage leaves, called cotyledons.
Once an embryo has emerged from its womb, eggshell, or seed, its main purpose is to grow. In children, larvae, saplings, and even young gametophytes and fern sporophytes, physiological resources focus on developing the size and skills that will be needed to thrive in mating play and child rearing. which follows, even if this role is limited to the accumulation of proteins. in an acorn.
For many creatures, the growth stage is at first glance similar to that of an adult, but just smaller. The onset of puberty, striking as it is for a human, has little effect on the overall body level. But for some, especially among insects, it can be surprisingly different. The specialized eating machine that is a caterpillar or a maggot, for example, can store energy through the use of an ecological niche that adults could not access.
In order for an adult to reproduce, and therefore pass on his genes, he must first find a mate. Sometimes the discovery is made directly by the sperm – fern sperm swim antheridia in which they form, through films of water, looking for archegonia that carry eggs of other gametophytes. Sometimes this is done by adults, through courtship rituals or competitions. Many flowering plants exploit an intermediary in the form of a pollinating insect, bat, or bird.
An oak tree bears its first acorns two or three decades after germinating, and may continue to do so for centuries. A human, after puberty, can look forward to decades of later life. However, many adult ages are brief. An extreme example is the mayfly, whose imagos cannot feed and exist only to mate and, if they are females, lay their fertilized eggs in the water they have just emerged from.
In animals, adults of long-lived species often care for their offspring and sometimes to some extent for others as well. It makes sense to collaborate with a close relative in raising children, as their children will carry some of your genes, but it can also make sense if the adults are not related, especially in situations where the favors are reciprocal. . It is believed that this social aspect of child rearing may explain why in a few species – humans and killer whales are notable examples – adults can live long enough after they have ceased to be able to reproduce.
Long or short, however, all lives are the same. The transmission of the genes of the body carried out (or not), the individual himself has no importance for the evolution. This explains why people not only age, but deteriorate. They evolved to be thrown away.
The inevitable mortality means that repairs and maintenance of the body do not need to be perfect, especially if the physiological resources necessary for them could be better used for mating and reproduction. Damage to cells in a body therefore gradually accumulates with age.
For animals, the transition from life to death, even when not administered by a predator, is rapid. The interdependence of an animal’s organs means that certain failures, especially failure of the circulatory system, are almost instantly fatal. A large plant, on the other hand, can die slowly because it lacks vital organs. Die, however, he will. But her offspring can survive, to fertilize once more with others of their kind. Biologists call these sets of inter-fertile organisms “species.” The slippery nature of this seemingly simple concept is dealt with in the next Biology dossier. ■
In this series on living standards
1 The large molecules of biology
2 cells and how to feed them
3 Making organs
4 The story of a lifetime *
5 What is a species, anyway?
6 Find living planets
This article appeared in the Short Schools section of the print edition under the title “Une danse sur la musique du temps”