La publicidad y la construcción de un público

Although Haeckel’s law proved to be a scientifi c failure, there were some grains of truth in it. Th ere are some similarities between human embryos and adult fi sh. Moreover, tetrapods almost certainly came into existence later than fi sh. So

the common ancestor of human beings and modern fi sh was in all likelihood something that looked much more like a modern fi sh than like human beings or any other tetrapod. Th is includes having gills, so we can say that, in so far as human embryos have things that look like gills, they have features that look like features possessed by our distant ancestors.

Much more important, however, is a fact that we noted in the previous section: human embryos, at an early stage in development, look very like the

embryos of fi sh at an early stage in their development. Similarly, at a slightly

later stage, human embryos no longer look like those of fi sh, but still look like those of other tetrapods. What this tells us is not that the development of human embryos follows the same path as human evolutionary history, but that in its earlier stages it follows the same path as many other creatures. Moreover, there is a pattern here: human embryos, as well as those of other chordates, look pretty much the same as those of bilateria in general, if you look at them early enough. Later, they no longer look like those of arthropods, annelids and so on, but they still look like those of chordates in general (including fi sh). Later still, they no longer look like those of fi sh, but still look those of tetrapods in general.

Th is fact is connected to another important feature in biology, that of homol-

ogy. Oft en we fi nd structures in common between diff erent types of creature,

especially so between ones that are closely related. For example, your arm has three diff erent kinds of joints: a ball-and-socket joint at the shoulder, a hinge joint at the elbow and a gliding joint at the wrist. You have the same three types of joints in your leg, at the hip, knee and ankle respectively. So there is a homol-

ogy – a common structure – between your arms and your legs. We can think of

arms and legs as variations on a theme. But this theme did not originate with human beings; the same types of joints can also be found, in the same order, in the legs of other mammals, reptiles and amphibians, as well as in the wings of birds. Th at is, it is common to tetrapods in general. If you look at the skeleton of a frog, you cannot help but be struck by how its front legs look like miniature arms. Th is is so even though a leg and an arm perform diff erent functions, and even though a wing performs a function diff erent from either. Sometimes, parts can be similar because they serve a similar function. For example, the Australian “marsupial mole” has front claws for digging that look quite like the front claws of moles proper, even though they are no more closely related to each other than either is to mammals in general. When similar structures are present because of similar function, they are called analogous, as opposed to homologous. Th e explanation for homologous structures is common descent: they are legacies from a common ancestor. Th us, the homologous structures in our arms and legs, frogs’ legs, birds’ wings and so on are legacies from early tetrapods.

Moreover, these homologous structures also have common developmental pathways. At an early stage on the development of a tetrapod embryo, append- ages begin to develop such that one could not tell just by looking at them

whether they were destined to be arms or legs or wings. We could think of this as a kind of “basic tetrapod limb”, from which all the various diff erent types of tetrapod limb develop. So fi nding out just how the innumerable variations of the basic tetrapod limb come about is a rich fi eld of study. Th e same can also be said of many other structures that can be seen to exhibit this theme-and- variations pattern. Th e appendages that extend from the bodies of arthropods are an extraordinarily varied lot, much more varied than tetrapod limbs. Th ink, for example, of insect legs, of which there is a surprisingly large variety. Or think of their wings, including, on many fl ies and mosquitoes, a pair of hindwings that are balloon-shaped and help with balancing. Th ere are also antennae, the claws of lobsters, the lungs of spiders. All of these – legs, wings, balancers, antennae, claws, and lungs – are now accepted by biologists as being variations on a common theme, in the same way that tetrapod arms, legs and wings are variations on a common theme.

One of the things that motivated this extraordinary realization about arthro- pod appendages was the discovery, in 1909, of a large collection of fossils in what is known as the Burgess Shale, in British Columbia, Canada. Not only did the shale contain an extremely rich diversity of diff erent creatures, but because of the way they were fossilized, scientists were able to study the soft parts of their bodies, not just the more usual bones, shells and so on. So scientists were pro- vided with a detailed snapshot of creatures that lived in the Cambrian period, about 540 million years ago. (Similar fi nds have been made in Sirius Passet, Greenland, and Chenjiang, China.) As one would expect, a host of creatures was discovered that looked nothing like anything living today. One creature struck palaeontologists as so strange it was named Hallucigenia.2 _{Th is sparked a debate}

about whether these creatures should be classifi ed as belonging to present-day phyla, or as belonging to diff erent phyla with no known modern representa- tives.3 _{Be that as it may, some of the creatures seemed clearly to be related to}

today’s arthropods. In particular, there were creatures called lobopods, because they had a large number of fat (lobe-like) legs. We might say that these stubby little legs were the basic arthropod limb of which the diverse legs, wings and so on are variations.

Th e combination of these discoveries with studies of embryonic develop- ment in insects and other arthropods led to the formulation of Williston’s law: in any lineage where there are serially homologous parts, the number of those parts tends to decrease, while the diversity and specialization of diff erent vari- ations tends to increase. “Serially homologous” just means that there are rows of structures that are homologous. Because arthropods have bodies made up of segments, their appendages are examples of serially homologous parts par

excellence. So the law is that earlier ones will tend to have many appendages that

are more or less alike, whereas later ones will tend to have fewer appendages, but of more diff erent kinds, and with more specialized functions. Th e evolution of

wings, antennae, lungs and so on from the simple legs of lobopods is a perfect example of Williston’s law. Like all “laws” in biology, it is a ceteris paribus law only: that is, there are many exceptions. Centipedes and millipedes, with their long rows of virtually identical legs, still fl ourish today. Nonetheless, the law is not vacuous, and it gives us some grounds for accepting the controversial idea that we can meaningfully speak of evolution having a direction.

We can also speak of this direction being echoed in the direction of devel- opment in an embryo. Th ere is a reasonably clear sense in which ancestral forms tend to be simpler than current ones, and embryos in the early stages of development tend to be simpler in the same way. Generalized buds in an embryo develop into specialized organs. Th is, then, is the grain of truth in Haeckel’s law. Moreover, the nineteenth-century idea of Bauplänen also seems to be making a comeback. Th e idea that many diff erent creatures’ body designs are variations on basic themes no longer seems as crazy as it did in the mid- twentieth century.

5.2.2 What can we learn from development?

What about the process of development itself? If our highly specialized and diversifi ed bodies develop out of much simpler embryos, very similar to the embryos of a host of other creatures, how do they do so? A human embryo at an early stage looks very like a dog’s embryo, and at an even earlier stage they both look like a fi sh’s embryo, and so on. But a human embryo still grows into a human being, not a fi sh or a dog. Why not? If these sound like the kinds of questions a child would ask, so much the better. Much of science (and philoso- phy) arises from asking childlike questions.

Th e obvious answer is: because our genes are diff erent from those of a dog or a fi sh. Th is is of course true, and it is an important part of the answer. A moment’s refl ection on some simple facts, however, will show that it cannot be the whole answer. Every cell in your body contains the same

dna

, with the exceptions of sperm and egg cells. But there are many diff erent kinds of cells, which perform many diff erent functions. Th ink of the diff erence between a nerve cell and a white blood cell, for example. Another childlike question is: how does a cell “decide” what kind of cell it is going to be? Th e complete answer cannot be “its genes decide”, because your nerve cells and white blood cells contain the same genes. Th e same conclusion can be reached by thinking about growing. Th e diff erent organs in your body all grew and then stopped growing, which means that cells multiplied and then stopped multiplying. So we can ask: how do they know when to stop? We know what happens when they do not stop: that is what a cancerous tumour is.

For a long time, development was treated as a “black box” in biology. It has been known since the 1950s that

dna

plays a hugely important role in shaping

an organism, and aft er the chemical makeup of

dna

was unravelled, much scientifi c eff ort went into discovering which genes were linked to which traits, and into mapping the entire genomes of human beings and other organisms. Th is remains one of the most signifi cant scientifi c projects of all time, but it led to those childlike questions being sidelined.

A great breakthrough in answering these questions was made during François Jacob and Jacques Monod’s research on the bacterium E. coli. Th is bacterium lives on the simple sugar glucose, but if glucose is not available it can take other sugars and break them down to produce it. For example, the more complex sugar lactose can be broken down into glucose and galactose. It does this using the enzyme beta-galactosidase. But this enzyme is only produced in any great quantity if lactose is present. So the question arises: how does E. coli “know” when to produce the enzyme? Scientists can identify which section of

dna

is “for” producing beta-galactosidase, but this gene only produces the eff ect when lactose is present. What Jacob and Monod discovered was a protein (called the lac repressor) that chemically binds to the section of

dna

that is involved in beta-galactosidase production. Th is prevents that section of

dna

from being transcribed into

rna

, and thus prevents the enzyme from being produced. When lactose is present, it binds to the lac-repressor, causing it to detach from the

dna

, so the enzyme is then produced. We can think of the lac repressor as a kind of on–off switch for the beta-galactosidase gene. Its operation, and therefore the eff ect of the gene, depends on conditions in the cell’s immediate surroundings. Th us we have an explanation for why diff erent cells do diff erent things, even though they contain the same

dna

. Two genetically identical E. coli bacteria, one in the presence of lactose and one not, will do diff erent things.

Many other on–off switches have been discovered in bacteria. Moreover, a similar type of mechanism has been discovered in multi-celled organisms, thus giving us an answer to the question of how a cell “knows” which type of cell to become. As ever, fruit fl ies were in the vanguard of this research. Studies of mutant fl ies with extra pairs of wings, or legs where their antenna should be, revealed that the mutations responsible were in specifi c regions of their

dna

, collectively named the “homeobox”. An alteration of a single codon in one of these genes gave rise to eff ects such as duplication of features (such as an extra pair of wings), or substitution of features (such as legs where antennae should be). It is as if the cells in certain parts of the fl y’s body “think” they are in a diff er- ent part. Th e cells where the antennae are supposed to be grow into legs instead. Why does this happen? Th e answer was found by looking at the protein that is encoded by the homeobox genes, which is called the “homeodomain”. In 1983, the geneticist Allen Loughran noticed that the homeodomain bore a striking similarity to the lac repressor in E. coli. It turned out that the homeodomain switches on and switches off various other genes in much the same way as the lac repressor does.4 _{Th us, depending on which part of a fl y a cell is in, certain}

genes will produce their eff ects and certain other ones will not. So we fi nally have a handle on the question of how cells “know” what type of cell to become. Depending on where it is, a specifi c subset of the genes in a cell will be activated, causing it to grow into a nerve cell, a white blood cell or whatever.

Homeobox genes and their associated proteins have been discovered in many other creatures, including diff erent kinds of worms, insects, mice, cows and human beings. Th e task of uncovering the full developmental story of any of these creatures will doubtless be a long one. But the discovery of the homeobox and the homeodomain have made it manageable, turned it from (in Noam Chomsky’s terms) a mystery into a problem. In doing so, it has given rise to a vast range of research programmes. By any reasonable standard, then, it is a major scientifi c achievement.

In document MEMORIA DEL TEATRO: HISTORIA DEL SALÓN MONTUFAR DE BUGA ( ). JHORJI STIVEN NARANJO COLMENARES (página 111-123)