4. Multicellular stage: embryological development. Higher levels of part-whole complexity in the structure of organisms are ontological causes of stages of evolution, and we have seen how three levels of biological organization are responsible for three stages of evolution (molecular, prokaryotic, and eukaryotic). It is obvious that there is a higher level of part-whole complexity than single-celled eukaryotes, for there are organisms composed of eukaryotic cells that are simply attached to one another. And a new stage of evolution does seem to have been caused by the appearance of one kind of multicellular organism, at least, for the Cambrian revolution some 600 million years ago, which is virtually the beginning of the fossil record, is the first evidence of multicellular animals. 

In order to prove the inevitability of another stage of evolution, however, we must show that a higher level of biological organization is both possible and can bring an entire new range of conditions affecting reproduction under control. Once again, the function of a higher level of part-whole complexity is obvious, because organisms are larger on the multicellular level and they can deploy the behavior of whole armies of lower level organisms. But it not so obvious how it is possible, because the capacity to coordinate the behavior of lower level organisms is more than simply bundling them together as parts of a higher level organism. It requires the capacity to coordinate their behavior (including their reproduction) to do non-reproductive work. As in the eukaryotic stage, therefore, the main challenge at the multicellular stage of evolution is showing the possibility of a biological behavior guidance system on a higher level of part-whole complexity.

Multicellular animals, rather than plants, will be our main focus, because they begin the series of evolutionary stages that lead up to beings like us. We shall focus, therefore, on the mechanism of embryological development in multicellular animals and even trace its origin. But then, in order to law the foundation for tracing the neurological stages of evolution in multicellular animals, we shall draw back and consider animals more generally, both the basic nature of animals and all the possible kinds of animals, basic and anomalous.

The possibility of multicellular animals. In order for organisms to evolve at the multicellular level, it must be possible for complex material structures to do both kinds of work required to go through reproductive cycles. One kind of work is reproduction, and at the previous levels, the higher level organism was able to reproduce by coordinating the reproduction of the lower level organisms of which it was part. But that is not possible at the multicellular level of biological organization, at least, not in animals in which the pace of evolution depends on sexual reproduction. 

Prokaryotes reproduce by replicating all the genes on their loop of DNA, and eukaryotes reproduce by replicating their chromosomes (and self-reproducing organelles in the cytoplasm). But what makes that possible is their biological behavior guidance system. In both cases, it is possible to coordinate the behavior of the lower level organisms, because the lower level organisms are parts of the biological behavior guidance system and they contribute to the behavior of the organism as a whole by how they move and interact within that more inclusive structure.

Multicellular organisms, by contrast, are just eukaryotic cells attached to one another, and since each contains all the structural causes of its own behavior, they would never be anything more than just colonies of eukaryotic cells, if multicellular organisms reproduced only by each of the cells reproducing itself. They could evolve such cooperative behavior by group level natural selection, but it would be very inefficient, like the evolution of RNA at the molecular stage and the colonies of prokaryotes from which eukaryotes evolved. What they need is a biological behavior guidance system to coordinate their behavior. But anything like the nucleus in eukaryotes or even the cell-enclosed loop of DNA in prokaryotes would be far too cumbersome. And even if it were possible, it would either have to give up the advantages of sexual reproduction or evolve a new way of sexually mixing the structures of its lower level organisms in the process of reproducing on the multicellular level. 

What makes it possible for multicellular organisms to have a biological behavior guidance system is that the cells of which it is composed can all be generated by the asexual reproduction of a single cell. And since it is generated from a single cell, multicellular organisms can evolve in the efficient way made possible by the sexual mixing of the lower level organisms of which they are composed, because the cell from which they are constructed can be a fertilized egg cell derived from different multicellular organisms in the previous generation.

At the multicellular level, in other words, the biological behavior guidance system is originally located within a single eukaryotic cell, and it is able to coordinate the behavior of all the eukaryotic cells making up the multicellular organisms because they are all its offspring and their behavior is coordinated from the beginning of their lives. This is what I will call the “mechanism of embryological development.”

Since the behavior of the eukaryotic cell is guided by the nucleus, this coordinating mechanism must be a modification of the nucleus. The plan for the behavior of every kind of cell in the mature multicellular organism must be contained among the chromosomes in the nucleus, and thus, a copy of the entire multicellular plan is bestowed on each daughter cell. But in order for each cell to generate its distinctive behavior, it must express only some of the genes on certain chromosomes, and that means that each cell must be able somehow to identify itself with one kind of cell in that plan, for otherwise the entire genetic plan would not affect different cells differently. A rather major modification of the nucleus is required to make the chromosomes in each cell express their genes in a way that is distinctive for cells of its kind, and it works only because the cells all come into existence in a unique way as offspring of the original fertilized egg cell

The mechanism of embryological development. The mechanisms of embryological development are only now being discovered, but in multicellular animals, at least, it must enable the nucleus of different kinds of cells to operate in basically different ways. Though such a mechanism is not problematic in the case of plants, animals need a more complex multicellular structure and that does pose a problem.

It is not difficult to see how plants could have evolved from single-celled eukaryotes. The mechanism responsible for constructing their multicellular organization is a simple variation on the eukaryotic behavior guidance system, which can reproduce asexually as well as sexually. In this case, daughter cells from the asexual reproduction of an original cell remain attached to one another and cells differentiate according to a gradient of messenger proteins (hormones) maintained throughout their extra-nuclear cytoplasms. Even a two-cell organism would be an advantage in completing a reproductive cycle, if one cell attached itself to the ground while the other grew out from it toward the sun. Not much of a change is required to establish a gradient from one end of the composite whole to the other, since single eukaryotic cell can already respond to molecules in the environment and orient themselves to electromagnetic radiation. What is required is a mechanism that would use the gradient of to open up the chromosomes of difference cells in different ways. Moreover, once it evolved, reproductive causation could easily come up with more complex plants, because during the whole process of development, the cells can be acquiring energy from photons. A complex process could evolve by adding step to a simpler process. In fact, plants probably evolved from plant-like protists many different times. For example, the spherical colonies of plant-like protists that float in the water (the Volvox series) appear to have evolved independently of plants that attach themselves to the ground with a holdfast.

There is, however, something quite puzzling about the evolution of multicellular animals, at least, in those that become most powerful, because they have bodies that are complex enough to require a nervous system to guide them. Their process of embryological development can also use gradients of messenger molecules to give cells different kinds of behavior, but something more is needed in the case of multicellular animals, because in order to set up a complex body, masses of cells (or even individual cells) must move in relation to one another in a regular pattern. Daughter cells cannot remain attached to one another in a fixed spatial relation.

Animals evolve from cells that are, unlike plant-like protists, capable of locomotion, and thus, coordinating them can involve coordinating their locomotion as well as other kinds of behavior. That means that the multicellular biological behavior guidance system in animals must be able to alter certain cells and their daughter cells early in the process so that, subsequently, they behave in ways that do not depend solely on the messenger molecules in the neighborhood (and can determine other cells they encounter to become cells of a specific kind as well). Once the structure of multicellular body is set up, the behavior of all the cells can be coordinated by the distribution of messenger molecules (hormones) throughout the body, because they will have different effects on different kinds of cells.

Moreover, embryological development in animals is not only more complex than in plants, but it is also quite puzzling how it could have evolved in the first place, because animals cannot acquire energy for themselves until the body is formed and they are able to ingest other living objects. The entire process of embryological development must be set up in advance with an adequate supply of energy in a fertilized egg, and the problem is how such an elaborate process could have been tried out as a variation during the gradual evolution of animal-like protists.

Such an elaborate coordination of animal-like protists cannot be simply an adaptation of the eukaryotic biological behavior guidance system, but requires a new structure in the fertilized egg cell that can construct a multicellular animal body from its offspring and coordinate their behavior. Such a process could be generated as a structural global regularity with just a few mechanisms.

There is evidence that what determines a cell to certain kinds of behavior rather than others are "homeobox genes", or master genes that activate or repress the expression of subordinate genes on nearby segments of their chromosomes.

And it is possible to alter cells (that is, “determine” cells) so that even their offspring have special kinds of behavior. For example, if what controls which segments (and, thus, which homeobox genes) are opened up were a self-forming complex of protein molecules that becomes attached to the DNA molecule at certain points, they could be reproduced when the cell reproduces, because one half of the protein complex could remain attached to each of the strands of DNA being copied so that subsequently, after it had synthesized its complementary strand of DNA, each half of the protein complex could attract the other half of the self-forming complex. Thus, daughter cells would have protein markers attached to its chromosomes that make them behave differently.

It is possible for the fertilized egg cell to determine its daughter cells at the very beginning so that they act, thereafter, in basically different ways from one another, independently of the hormones that they may encounter, because different daughter cells inherit different parts of its cytoplasm. If different messenger molecules were located in different parts of the fertilized egg cell, they would wind up in different groups of cells after the first few divisions, and the process of embryological development would begin with different groups of daughter cells having basically different kinds of behavior, including locomotion relative to one another.

Even though a multicellular animal is, at one point in its reproductive cycle, just a single cell (a fertilized egg cell), it must be counted as a higher level organism, because that cell is the structural cause of embryological development. Embryological development is a complex structural global regularity whose outcome is a complex organization of various kinds of cells in which the behavior of the cells is coordinated by the exchange of messenger molecules. And since the complex material structure responsible for this global regularity goes through reproductive cycles with both essential kinds of behavior, it evolves by reproductive causation toward natural perfection for organisms of its kind.

To be sure, multicellular animals are different from prokaryotes and eukaryotes. Prokaryotes and eukaryotes have the same level of organization throughout their reproductive cycles, whereas multicellular animals have a period during which they are at the eukaryotic level of biological organization. But that merely reflects the nature of their biological behavior guidance systems. In prokaryotes and eukaryotes, the biological behavior guidance system is the an aspect of the structure of their highest level of biological organization, whereas in multicellular animals, the basic structural cause is located in a single cell, the fertilized egg cell. But since it is clearly the structural cause of a multicellular organism, there is no reason to doubt that it is on a higher level of biological organization.

Levels of biological organization are basically levels of part-whole complexity in space, but as we have seen, they also typically involve levels of part-whole complexity in time. That is, a single reproductive cycle at the higher level may include many reproductive cycles of the lower level organisms of which they are composed. Prokaryotes and eukaryotes both use many cycles of the RNA molecule’s reproduction in doing their non-reproductive work, because it depends on transcribing segments of the their DNA as mRNA and then using them to guide protein synthesis. And in eukaryotes, mitochondria and chloroplasts may go through many reproductive cycles during a single reproductive cycle of the cell as a whole. In addition to incorporating lower level reproductive cycles, a single reproductive cycle of a multicellular animal uses many eukaryotes reproductive cycles to construct a multicellular body. The main difference is that multicellular animals use many cycle of the reproduction of the lower level organisms not only to do non-reproductive work, but also to reproduce themselves. But that is not a reason for doubting that they are on the multicellular level of biological organization.

These general points about its nature show how a new kind of biological behavior guidance system is possible, in principle, at the multicellular level. That is, there is little reason to doubt that the mechanism of embryological development would eventually be tried out as a random variation during the evolution of eukaryotes. Since the greater power of multicellular animals is hardly in doubt, we can conclude that the multicellular stage of evolution was inevitable.

The section at the end of this chapter uses what is known about the simplest multicellular animals on earth to explain how the origin of the mechanism of embryological development can be traced to single-celled eukaryotes.