As we saw in Chapter 2, there have been multicellular life-forms on this planet since about 1200 million years ago (MYA). These first life-forms, however, were not animals; they were probably red algae. Later, in the Ediacaran period, which began some 635 MYA, there are fossils of multicellular creatures, some of which may have been animals; their interpretation remains controversial. The beginning of the Cambrian period, 542 MYA, marks the start of an abundant fossil record of creatures that undoubtedly are animals. In the Cambrian we are faced with a profusion of animal fossils, both those that we recognize as being clearly related to some of today’s animals, and others that are more enigmatic in terms of where to place them in our ‘groups within groups’ system of naming and ordering animals that we inherited from Linnaeus.

Geological periods are often named after places where rocks of the relevant age are found. As we noted in Chapter 2, the Ediacaran period is named after the Ediacara Hills in Australia. The Cambrian is named after Wales. The basis of this latter naming is not as readily apparent as that of the former. But the Welsh name for Wales – Cymru – gets us a bit closer to seeing the connection. And the Roman name – Cambria – makes it crystal clear. There is even a town in Wales where the local newspaper is called The Cambrian News.

When first examining animal development in Chapter 13, we noted that a pioneer of the subject of embryology was the Italian Hieronymus Fabricius, whose studies on the development of the chick were published posthumously in 1621. This work can best be considered as descriptive embryology – it involved a focus on a particular animal and gave as detailed a description of its embryogenesis as the techniques of the time would allow.

It was not until about two centuries later that embryology became truly comparative. This aspect of embryology is clearly related to evolution, as was recognized by Darwin, who, in 1859, devoted part of chapter 13 in On the Origin of Species to their relationship. An often-quoted statement that Darwin makes there is: “community in embryonic structure reveals community of descent.” We might do well to examine this statement carefully, to try to come up with a modern version of it, and to see what course of logic such an attempt will set in train.

Although the subject matter of this chapter is very different from that of the last one and the next one, a common theme running through all three is probability, or perhaps – though it’s really the same thing looked at from a different perspective – improbability. In the last chapter we saw that the nature of the developmental system made the evolution of certain kinds of animal improbable. In the next chapter we look at the probability of animal-type life on other planets. Here, we look at animals called extremophiles that live in parts of our own planet where animal life would at first sight seem improbable, and/or that have tolerance of extreme conditions.

Although these animals are indeed called extremophiles, this term is used also to describe other forms of life, from other kingdoms, that can withstand extreme environments. So the term can be used also for plants and fungi, and especially for organisms from the bacterial and archaean domains, where an extremophile existence is most commonly found. But here we’ll concentrate on extremophile animals. We’ll also concentrate on extremes of temperature; but it’s worth noting that extremophile can be used in relation to other environmental variables – for example acidity.

Every animal gets made by two processes, which take very different lengths of time. The longer-term process is the one we’ve already been discussing in most chapters of the book: evolution. Here, a particular type of animal is made from a different, earlier-arising type by a series of modifications that rely on Darwinian natural selection and perhaps, as we’ll see later, on other things. The shorter-term process is the one we will now begin to address explicitly: development. Here, an animal is made from the starting point of (usually) a fertilized egg.

Although evolution and development work on very different timescales, they are inextricably linked. Each is, in a manner of speaking, the starting point for the other. To see this clearly, it helps to consider the whole of egg-to-adult development as a trajectory, or, to put it another way, as a route from a simple, unicellular beginning to a complex, multicellular end. Each type of animal has such a trajectory, though when animals with very different adult forms are compared, their developmental trajectories are found to be likewise very different (especially in their later stages). For example, although we have not looked at any developmental details yet, it is clear, to use the molluscs of the last chapter as an example, that a very different route must be taken from the fertilized egg to end up in one case with a snail and in another case with an octopus.

A pervasive theme that has been with us since the beginning of the book – sometimes explicitly, other times implicitly – is that more complex animals are not necessarily more ecologically successful than simple ones. Indeed, this theme first emerged (Chapter 1) in a broader context than the animal kingdom – the context of life in general. The very first life-forms on Earth were probably rather like today’s bacteria. The fact that bacteria and other unicellular forms continue to prosper attests to the fact that you don’t have to be a big, complex organism to be fit – in the evolutionary sense of the word, as explained in Chapter 5. This point was reinforced in the previous chapter when we noted the incredible ecological success of roundworms, most of the 25,000 species of which are small and, in structural terms, quite simple, compared, for example, to arthropods or vertebrates.

It was this point (among others) that led me to state, at an early stage in the book, that evolution is not an escalator, up which creatures go at varying rates to higher levels of complexity. Some evolutionary lineages have shown rises in complexity over geological time, but others have not. Some have even shown decreases in complexity.

Evolutionary trees, such as those we looked at in the last chapter, are very abstract things. They take a complex process involving many animals, molecules, morphological characters, matings, migrations, speciation events and geography, and render this multi-level, four-dimensional complexity into a simple-looking picture on a single sheet of paper. For some purposes that’s great. After all, when we are trying to understand a complex process, any device that captures the essence of the process in a simple diagrammatic way is incredibly helpful. But it can also be misleading; and whether it does indeed capture ‘the essence of the process’ can be questioned. I think this is especially true in relation to the extinct animal that has come to be called the urbilaterian (and nicknamed, somewhat tongue-in-cheek, ‘Urbi’).

What was this strangely named beast? Answers to this question can come in a variety of forms. In terms of the name, it uses the German prefix ur-, meaning first or original, and couples that with bilaterian, which is short for ‘bilaterally symmetrical animal’. As we saw in previous chapters, the most basally branching animals either had little or no symmetry (sponges) or had radial symmetry (jellyfish and their kin). But most other animals that we see around us are bilaterally symmetrical, albeit with various degrees of imperfection. Therefore, at some point in the distant past, bilateral symmetry must have originated, and the first animal displaying it was the urbilaterian.

Charles Darwin’s On the Origin of Species, published in 1859, can rightly be regarded as the start of evolutionary biology. That’s not to say that it was the first publication on evolution, but it was the first to convince most scientists – in some cases immediately and in others eventually – that (a) evolution had happened and (b) it occurred via a particular mechanism, namely natural selection, or ‘survival of the fittest’.

In the previous chapter we saw that, whatever uncertainties remain about the origin of animals, by 500 million years ago the Cambrian oceans were teeming with animal life. They were doubtless teeming with bacterial and algal life too, though the flowering plants that dominate the plant kingdom today did not evolve until much later. In the Cambrian, all multicellular life-forms were aquatic – the land did not get colonized by plants and animals for probably another 100 million years.

Because natural selection is a general mechanism of evolutionary change, it must have been operating in those ancient marine ecosystems of the Cambrian much as it operated in their more recent terrestrial equivalents over the last six or seven million years to modify human and chimp lineages from their last common ancestor. And indeed it is still operating in the same way today, as we see in the evolution of biocide-resistant insects and bacteria.

Consider a typical garden somewhere in Ireland – or, for that matter, in Britain, France or Massachusetts. Regardless of its exact size, many animals will die there on a daily basis. Most will leave no trace of their existence; within a few days or weeks it will be as if they had never lived. Far from becoming fossils for palaeontologists of the distant future to inspect and interpret, they will leave no clues as to their structure, function, or ecological context.

A good example is the death of an earthworm when a blackbird pecks it out of the top layer of the soil and eats it whole. Not only will the worm’s flesh be completely digested in the alimentary canal of the bird, but earthworms have no hard parts – no teeth, shell or bones – to be egested by the bird and left on the ground for possible fossilization.

In the introductory chapter, we looked at the structure of the animal kingdom in terms of the relative numbers of species belonging to different groups. There, we concentrated in particular on the relative numbers of vertebrate and invertebrate species; and we saw that vertebrates make up less than 5% of the animal kingdom. Here, we pursue the theme of the make-up of the animal kingdom further, but from a different perspective: not the proportion of it that is vertebrate but the proportion that is vermiform – or worm-like.

However, this issue is not as simple as it seems. There are many different ways of assessing the size of any particular group of animals. Also, as we have seen already, there are many different ways of defining ‘group’, especially in the light of Darwin’s ‘groups within groups’ picture, for which he gave a mechanistic explanation – evolution. Prior to Darwin, the groups-within-groups view had probably been held by all serious students of zoology, and had been formalized by the Swede Carl Linnaeus – for plants as well as animals – way back in 1735.

We can generalize the point made in the last chapter about the role of environmental factors in determining human height to the role of these factors in determining the body sizes of most if not all animals. Indeed, we can generalize further, because environmental factors play a role in determining the values of many characters in most or all animals, including those that are unrelated to size. In other words, the genes do not completely determine the course of development; rather, this course is set by a mixture of genetic and environmental influences, and the interactions between them.

There are several ways of describing the effects of environmental factors on development. One way is to say that a character such as body size upon whose value there is an environmental influence has only a partial heritability. Another is to say that there is an element of plasticity in the character concerned. More complete terms for the latter are phenotypic plasticity or developmental plasticity, the second term being preferable for reasons already given in Chapter 22.

The great American palaeontologist G. G. Simpson, in discussing Darwinian and other evolutionary theories, made the following point in 1953: “The various major schools of evolutionary theory have arisen mainly from differences of opinion as to how evolution is oriented.” Of course, in the Darwinian school, natural selection is seen as the main cause of evolutionary orientation, or direction. In other words, this school of thought has it that natural selection steers evolution and thus causes particular lineages to go in one direction, say increasing body size, rather than another. This differs from the earlier Lamarckian approach, which involved the inheritance of acquired characters – now known not to occur – in the determination of evolutionary direction.

Natural selection is a systematic rather than random process. But its raw material is variation, and this derives ultimately from genetic mutation, which is often said to be random. However, this latter point needs some elucidation. A key assumption of Darwinian theory – sometimes explicit, sometimes implicit – is that the variation upon which natural selection acts is random with respect to whatever would be favoured under the prevailing environmental conditions. In other words, the variation is considered not to be biased in favour of forms that would have increased fitness. This is generally believed to be correct.