The case of the kiwi is a useful reminder that despite sticking to eggs, many other non-mammalian vertebrates have adopted the system of ovoviviparity, in which the egg is retained in the mother prior to a live birth. The squamate reptiles (lizards and snakes) are particular masters of ovoviviparity: Richard Shine estimates that in this group this type of reproduction has evolved independently approximately 100 times,141 while Daniel Blackburn suggests that overall viviparity has evolved independently approximately 132 times,142 with examples also in the anuran amphibians143 and fish.144 Not surprisingly, the retention of the embryo in the female reproductive tract has led to other convergences, most notably the emergence of a placenta.145 Particularly remarkable in this respect is the similarity between the mammalian placenta and an ovoviviparous Brazilian lizard (Mabuya heathi) which, as Blackburn and his colleagues remark,146 'has strongly converged upon a reproductive pattern long believed to be unique to eutherian [i.e. placental, as against the metatherian marsupials] mammals'.147 The relevant features, in addition to the placenta and its nutrient supply, include a remarkably small egg (c. 1 mm), comparable in size to a typical mammalian ovum. Not surprisingly the period of gestation, for a reptile, is prolonged. This has the curious side-effect that females can be impregnated while juvenile and give birth to their live young when adults. Other species of the lizard Mabuya show similar features,148 and it is possible that such viviparity originated at least four times in this group.149 Other viviparous lizards show interesting features convergent with mammals. In a European species of Chalcides (C. chalcides) the placentome, a specialized area involved with absorption, develops to such an extent that at the time of birth it can be cast off, and thus uniquely for the reptiles is equivalent to the mammalian afterbirth.150 A curious parallel also exists in the spade-nose shark. Here, too, the ovum is minute (c. 1 mm), and quickly implants in the uterine wall where it receives nourishment through a placenta-like structure. The combination of tiny egg and major maternal input again leads to a massive increase in the weight of the developing young.151
In reviewing the occurrences in the lizards, Daniel Blackburn emphasized that 'In several important respects, matrotrophic Mabuya and Chalcides have converged strongly with mammals. Features that have evolved independently among eutherian mammals and in one or both of these two reptilian groups [total at least seven],'152 and he concludes that
Classic textbook examples of convergence, such as the evolution of flight, and fusiform body shapes in aquatic vertebrates, pale beside those relating to viviparity and matrotrophy. Among viviparous vertebrates evolutionary convergence has operated at virtually every level - not only with respect to viviparity itself, which has originated over 130 times, but at the level of the organ, the tissue, and the cell. Such convergences are all the more striking when we consider that the historical sequences through which viviparity and matrotrophy have evolved apparently differ from lineage to lineage. Moreover, the independently-derived
222 alien convergences?
specializations for fetal nutrition refute theriocentric [i.e. mammal-centred] views of mammalian reproduction and placentation as uniquely specialized.153
Blackburn's analysis epitomizes many of the important features of convergence, especially the degrees of similarity and the fact that the end-result may be arrived at by various historical pathways. Another important facet of convergence is emphasized in a study of sharks and rays, where viviparity (see also note 151) is estimated to have evolved independently at least nine times. In at least this group there is a very strong polarity towards viviparity, with only a few counter-examples.154 It is worth noting, however, that the strong phylogenetic trend to viviparity is not matched by the evolution of matrotrophy (nutrition provided by the mother in addition to the yolk), which evo-lutionarily is much more labile. This has evolved independently four or five times, but also with multiple reversals.
In the context of non-mammalian ovoviviparity, there is one other item worth mentioning. Earlier (Chapter 5, p. 96) I suggested that as a contingent episode the asteroid that hit the Earth 65 Ma ago, and so led to the demise of the great reptiles, which thus facilitated the subsequent rise of mammals, may be misconstrued. Not as a historical episode: there seems little doubt that the mammalian radiations are a direct consequence of the disappearance of the Mesozoic reptiles. That, however, does not address the question that concerns the likelihood, if not the inevitability, of the emergence of such biological properties as 'mammal-ness'. Viewed in this light a different picture emerges, and for two reasons. First, as the Tertiary planet cooled, culminating of course in the ice ages, the mammals (which, recall, appeared about 220 Ma ago, at about the same time as the dinosaurs) would have been at an advantage anyway, well adapted to temperate and even polar environments. This suggests that the rise of active, agile, and arboreal ape-like mammals, and ultimately a hominid-like form, would have been postponed, not cancelled. My guess is that without the end-Cretaceous asteroid impact and in due course progressive planetary refrigeration, the appearance of the ho-minids would have been delayed by approximately 30 Ma. But even that is a local history. It is interesting that, at least in the lizards, the evolution of viviparity is associated with colder climates.155 It is not the only factor, but recalling also that 'honorary mammal' the kiwi, it too suggests that 'mammal-ness' will not be the exclusive property of one evolutionary group. If 'we' had not emerged, then (as this and the next chapter aim to show), rest assured that a viviparous, warmblooded, vocalizing, and intelligent species would have done so.
Finally, as if it needed any emphasis, it is important to stress, as Nicholas Dulvy and John Reynolds specifically remark with respect to shark and ray viviparity 'Our data do not support a linear, irreversible progression toward a 'pinnacle' of maximum maternal input.'156 Quite so; evolution is labile, it does show reversals, but the point still remains that the emergence of various biological properties is in response to adaptation and is governed by selection. Nor does this mean that the selective pressures operating in an ocean need be the same as on the savannah, although as we shall see later (p. 250) here, too, there are some surprising convergences, specifically between sperm whales and elephants. Convergence simply tells us that the evolution of various biological properties is certainly highly probable, and in many cases highly predictable.
warming to convergence, singing of convergence, chewing convergence The stories of the kiwi and non-mammalian ovoviviparity are two reminders that being a mammal and evolving towards 'mammal-ness' need not be the same thing. There are at least two other features that typify mammals but are in fact also strikingly convergent: warm-bloodedness and vocalization. Concerning the first, endothermy (that is, the phenomenon of having a body temperature elevated in comparison with the ambient environment), is rampantly convergent and occurs not only in the birds and mammals, but also in various fish (teleosts and sharks) and even in insects.157 The example of the fish might seem somewhat removed from the path of humanoids, but may in fact have some relevance. In the fish endothermy is clearly convergent, and has probably evolved on at least five separate occasions. Examples include the remarkable modification of the eye muscle in some scombrid fish, to form a heater to warm the brain and retina (see note 118, Chapter 7). Whole-body endothermy in fish is perhaps best known in some tuna, famous for their fast swimming and wide ranges. Barbara Block and her colleagues158 stress that this is a sophisticated adaptation, and entails a concatenation of changes. These include the development of countercurrent heat exchangers at three locations in the body, the movement of red muscles towards the interior of the animal, combined with the body becoming larger and thicker, and a style of locomotion known as thunniform, in which swimming is achieved by oscillation of the tail rather than sinuous deformation of the entire body. Yet this complex arrangement was convergently arrived at in the lamnid159 and alopiid sharks, where again it probably evolved independently in each group.160 There are, moreover, more specific convergences161 such as the 'Striking parallels in both structure and function [that] exist between the shark orbital rete [a fine network of blood vessels] and the mammalian carotid rete which serves as a brain cooling system in mammals'.162
The adoption of this type of endothermy is significant because it evidently allowed an expansion of a habitable zone, and in these fish it made possible the invasion of a vast new realm, specifically the cooler (and deeper) parts of the ocean. As Barbara Block and her co-workers note, the multiple origins of endothermy indicate 'Strong selection for this energetically costly metabolic strategy'.163 In other words, en-dothermy is expensive, but well worth the cost by being highly adaptive and accompanied by success. Costs, as ever, have risks, and in the context of endothermy there is another interesting physiological convergence. This involves a pathological condition known as malignant hyperthermia, in which the muscles produce excessive heat that if unchecked leads to severe tissue damage and death. In mammals it has a genetic basis, and may manifest itself in anaesthetized humans undergoing surgery, as well as in genetically disposed pigs that panic in the slaughterhouse.164 A strikingly similar syndrome is found in highly stressed tuna, especially those caught on a hook and line. This leads to so-called 'tuna burn', badly damaged musculature and subsequent rejection by the fastidious Sashimi enthusiast.165
What then is the significance of the convergent evolution of endothermy in the birds and mammals? C. G. Farmer has argued that among the welter of possibilities the key factor is parental care.166 Of central importance is the need for sustained exertion, to enable the parent(s) to collect sufficient food not only to maintain their ravenous fledglings or litter, but also for them to grow as quickly as possible and thereby decrease their vulnerability. Even in those birds and mammals that characteristically save energy by entering a state of torpor and letting their body temperature decline, incubating birds and pregnant mammals typically retain a higher body temperature. Farmer suggests that the original trigger for bird and mammal warm-bloodedness was connected to reproduction and the increased production of hormones, e.g. from the thyroid, which in turn are also important determinants of metabolic rates. She continues by noting that
The convergent evolution of ... parasagittal limb posture ... a large pulmonary diffusion capacity ... high oxygen-carrying capacity of the blood ... a large tissue diffusion capacity ... a completely divided cardiac ventricle ... an extensive coronary circulation and compact myocardium ... high systematic blood pressure ... and a single aorta ... may be due, at least in part, to selection for an ability to sustain vigorous exercise which is requisite for birds and mammals to provision their young.167
Parental care is, of course, a particular hallmark of the birds and mammals,168 and Farmer concludes that this in turn may have promoted both 'complex social structures [and] ... vocal signaling'.169 And that might seem to be the furthest these convergences can be pushed. Birds are warm-blooded, indeed generally showing significantly higher body temperatures, and of course they can also sing. Is the ability to sing only a vague similarity? Not a bit of it; now it is time to see just how far vocalizations are also convergently arrived at.
The ability of birds to vocalize needs, of course, little emphasis: twittering, booming, squawks, hooting, and melodious song are all familiar, as are the mimics, such as some parrots and the mynah birds. One African Grey I knew could produce a remarkable repertoire of sounds, including the Greenwich time pips with eerie accuracy and now, her companion Caroline Pond tells me, is well up with mobile phones and car alarms. And there are more poignant instances. Darwin, for example, in a section on the extinction of races and tribes remarks that 'Humboldt saw in South America a parrot which was the sole living creature that could speak a word of the language of a lost tribe.'170 Even so, we tend to regard bird mimics as simply amusing, and their songs as only incidentally beautiful and in reality vocal expressions of territorial demarcation and the urgent priorities of mate attraction. Perhaps so, but it transpires that in reality there are under-appreciated similarities between our vocalizations and those of the birds, both in terms of songs and musical output171 (and even drumming with a stick172) and neurology. Thus, with respect to song, both birds and humans share such features as 'interval inversions, simple harmonic relations, and retention of melody with change of key', while our 'simple melodic canon ... is reminiscent of the matched countersinging of many bird species.'173 So, too, claims of human uniqueness with reference to such matters as the voluntary control of the supralaryngeal tract, a prerequisite for human language, in reality finds parallels in African Grey parrots.174
So far as neurology is concerned, the behavioural biologists Allison Doupe and Patricia Kuhl remark,175 concerning birdsong and human speech, that 'there are striking similarities in how sensory experience is internalized and used to shape vocal outputs, and how learning is enhanced during a critical period of development', and they continue, 'similar neural mechanisms may therefore be involved'.176 Significantly, Doupe and Kuhl use the phrase 'strikingly similar' a number of times. Thus, in referring to the vocal control systems in songbirds and humans, they comment how continuing study 'reveals numerous anatomical and functional similarities in the organization of neural pathways for vocal production and processing ... both fields [of enquiry] grapple with strikingly similar questions about how sensory and motor processes interact in vocal learning and production,'177 while concerning the learning and self-organization of sounds they remark that the 'perceptual patterns stored in memory serve as guides for production [and are] strikingly similar'178 in birdsong and human speech.
In some ways the similarities of these various processes are all the more surprising, given both the various differences in avian and mammalian brain structure, e.g. the absence of a multi-layered cortex in the avian brain, and in some species of bird strong sexual dimorphism of song production. Yet the similarities, striking or otherwise, still emerge. Even so, Doupe and Kuhl are careful to qualify these remarks, noting that although the parallels are striking there are also a number of obvious differences, most notably the human possession of a grammar (but see Chapter 9). Yet what they rightly call the 'numerous parallels'179 between my remarking to my companion on the beauty of a bird's song and the song itself, suggest that not only warm-bloodedness and viviparity but also at least some mechanisms of both vocalization and song may be widespread across the Galaxy (see note 132, Chapter 9). So, too, given the recurrent emphasis on evolutionary convergence, it is not surprising to learn that in the birds the powers of song and vocalization have evolved at least twice.180
Song, the birth of live young, and warm-bloodedness, not to mention that honorary mammal, the New Zealand kiwi, all provide compelling examples of convergence that show how features we associate with the mammals are more widespread and may, therefore, reasonably be expected to emerge elsewhere. Nor do the examples end here. Mammalian dentition is well known for its complexity. Earlier, we encountered the convergent evolution of the massive canines in the sabre-toothed cats and marsupial thylacosmilids. In addition to the canines, most mammals have nibbling teeth (incisors) and the familiar battery of molars for grinding or shearing.181 That this dentition makes sophisticated food processing possible has no doubt contributed to the evolutionary success of the mammals. In contrast, the dentition of the reptiles is much simpler. Typically it consists of a sharp array of pointed teeth, and if there is any difference along the length of the jaw it is usually only one of relative size. Exceptions, however, are known, and they show striking convergences with the dentition of mammals. Not only that, but this type of dental convergence has arisen at least twice,182 in two distantly related groups of reptiles, both of which lived in the Cretaceous. The first concerns a teiid lizard, from sediments close to the Cretaceous-Tertiary (K/T) boundary in Montana. Here the posterior teeth on both the lower (dentary) and upper (maxillary) jaws have become molariform, and resemble the so-called tribosphenic arrangement of mammalian molars with its series of interlocking cusps.183 In addition, the method of regular tooth replacement, whereby old and worn teeth are discarded and new ones take their place, which is the norm in reptiles, is suppressed.184 As in the mammals, after the loss of the deciduous (or 'baby') teeth, the next set is for life. The second example concerns some crocodilans from China and Malawi, whose dentition has again ground towards the molariform solution. In these examples chewing appears to have been somewhat less effective in its action, without direct occlusion. Unlike the teiid lizard, which probably ate insects, these crocodiles were vegetarians.185 While considering the somewhat improbable topic of plant-eating crocodiles, it is surely worth mentioning in passing another example from the fossil record of a group of crocodiles (the ziphodonts) that became effectively fully terrestrial and in doing so developed something rather like hooves.186 And are crocodiles unique? Well, not really, or at least so far as the skull is concerned. The spinosaurs,187 hitherto regarded as a rather enigmatic group of theropod dinosaurs, have a skull that is interpreted as a crocodile mimic.188
By now, I hope it will be clear that many of the evolutionary features that help to define the human are convergent. If such features as warm-bloodedness, vocalization, and even agriculture can evolve independently, then so, too, on any suitable planet the same will emerge. Yet at this stage there is surely a dimension missing. To be sure these and many other convergent features serve to delineate complex biological systems, but the scope is still very wide-ranging and encompasses animals as disparate as ants, tuna, and kiwi. Even if we grant that 'mammal-ness' is a biological property rather than a historical contingency, we still seem to be far removed from anything specifically human with such hallmarks as bipedality, tool-making, culture, and intelligence. Let warm-bloodedness, vocalization, and even agriculture be convergent, but surely the hallmarks of the human are simply the quirky results of contingent happenstance. On Threga IX there will be much that is reminiscent of Earth, but - so it is widely believed - any consciousness will be submerged in the inarticulate and any music will be little more than harmonic babbling. The opposite turns out to be the case.
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