Science: That's no bat, that's my brother: Geneticists claim to have all the answers when it comes to fathoming our relationship to other animals. Things are not that simple, says Colin Tudge

No issues have stirred the peaceable waters of zoology more violently these past 200 years than those of systematics: which animals are related to which. The trouble is that animals that look alike often have different ancestry - as with dolphins and fish, which merely provide an example of 'convergent evolution'. But sometimes animals that look different have both descended from a common ancestor - as seems to be true of condors and storks, even though condors look like ordinary vultures.

Many such matters remained unresolved (and controversial) until, in the Seventies, molecular biologists swept in like the US cavalry and showed that relationships could be revealed unequivocally by studying the chemistry of genes. It seemed necessary merely to compare the structure of the DNA. Such studies showed that humans are almost the same genetically as chimpanzees, and that the two probably diverged no more than 8 million years ago.

But zoology would be boring if it was that easy. That the DNA studies themselves require all-too-human judgement has long been suspected, and is now made clear through creatures that have puzzled systematists since the 18th century: bats, and their relationship to Primates - that is, to lemurs, tarsiers, monkeys, apes and ourselves. John Pettigrew of the University of Queensland argues that fruit bats, which include 'flying foxes', effectively are Primates and are not related to 'ordinary' bats at all. To be sure, fruit-bat DNA seems very like the DNA of, say, pipistrelles, and nothing like that of monkeys. But, says Dr Pettigrew, DNA itself is subject to the vagaries of evolution, and in this instance, as perhaps in many others, the DNA is lying.

Zoologists generally place bats as a whole into the 'order' Chiroptera - but they divide that group into two sub-orders. First come the Megachiroptera, or 'megabats', which include 162 species of fruit bat including the huge 'flying foxes' with a wingspan of a metre or more. Then there are the Microchiroptera, or 'microbats', with 815 species of pipistrelles, Daubenton's, horseshoes and the rest. There is a general difference in size between the groups (though much overlap) but the similarities are obvious.

Dr Pettigrew, however, points to at least 50 differences. Microbats, for example, use echo location for hunting but megabats do not - or if they do, employ a different, cruder system. The fossil record shows that microbats evolved before megabats, so if the two are related

then megas must have arisen from micros. But, if so, why did megabats lose echo location, which would have been useful for them since they, too, forage at night? And why, having abandoned echo location, did some megabats re-evolve their own system?

Instead, Dr Pettigrew points to the similarities between megabats and yet another order of mammals, the Dermopterans, otherwise known as colugos. These are gliders from South-east Asia and look so like Primates that they are often called 'flying lemurs'. Indeed, Linnaeus argued in the 18th century that colugos were Primates, while modern zoologists generally agree that Dermoptera and Primates are closely related. Indeed, the fossils suggest that colugos descended from Primates.

In practice, some of Dr Pettigrew's suggested resemblances between megabats and colugos, such as the wing structure, could be due to convergence. But others seem to reflect a relationship - including many details of brain anatomy, such as the way in which particular groups of cells are packed. Such obscure features are not readily affected by pressures of the environment and are usually ascribed to common ancestry.

There is one anatomical complication. Some zoologists point out that colugos have such small brains that they could not be related either to Primates or megabats. But, says Dr Pettigrew, this is because colugos eat the leaves of tropical trees. Such leaves are high in toxins and committed eaters of tropical tree leaves tend to have small brains for this reason alone, irrespective of ancestry. Koalas and hoatzins (primitive South American birds) have similarly pea-sized brains. Never mind the size, says Dr Pettigrew, look at the structure.

In fact, he says, megabats are probably descended from ancient colugos, since colugos are more primitive and have an older fossil record. But if megabats are descended from colugos, and colugos from Primates, then megabats in effect are Primates. Microbats, on the other hand, as most zoologists agree, had insectivore ancestors.

Dr Pettigrew's theory gained ground in the mid-Eighties - until the molecular biologists came on the scene. They showed that megabat DNA and microbat DNA were very similar. And both were distinct from that of Primates.

But, says Dr Pettigrew, the resemblance of megabat and microbat DNA is itself due to convergence. DNA is compounded from four bases: adenine, thymine, guanine, and cytosine (A, T, G, and C). Megabat and microbat DNA are both rich in A (and therefore T, which always accompanies A). Why should this be, if common ancestry is not the cause? Well, A, adenine, is very like adenosine. Adenosine is the basis of the molecule ATP. ATP provides cells with energy. So animals tend to have a high concentration of ATP if they have a high metabolic rate and therefore a high demand for instant energy.

Now the twist. DNA - which is always being copied and otherwise abused - is in constant need of repair. Second, only a small proportion of the total DNA molecule functions as genes, the rest seems almost functionless. Repairs to the functional bits have to be accurate but repairs to the rest - the majority - can be more rough and ready. In the cells of animals with a high metabolic rate, there are huge amounts of adenosine, so this base, albeit slightly modified, is used for DNA repair. Thus animals with a high metabolic rate have DNA that is rich in A (and therefore inevitably in T). All bats, as flying animals, have a high metabolic rate, and thus their DNA is rich in A and T. So, too, do shrews and honey- bees - which also have a high metabolic rate. But, says Dr Pettigrew, if you take away the A-T bias, which is brought about by the high rate of metabolism, you find that megabat DNA is more like that of Primates and colugos than that of microbats.

Further studies - for example on the DNA of more animals of high and low metabolic rates - must explore Dr Pettigrew's arguments. For my part, I feel he is right: that mammals produced not one, but two distinct groups of powered fliers. In any case, his general argument is salutary. DNA does not provide the royal road to unequivocal insight. Systematists must use their judgement in assessing DNA, just as they do when looking at any other feature - behaviour, brain structure or bones.

Colin Tudge's latest book, 'Engineer in the Garden', on modern genetics, is published by Jonathan Cape, pounds 17.99.

(Photograph omitted)

Suggested Topics
Start your day with The Independent, sign up for daily news emails
ebooks
ebooksAn introduction to the ground rules of British democracy
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
SPONSORED FEATURES
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Barnardo's: Corporate Audit and Inspection – Retail Intern (Leeds)

Unpaid - £4 lunch allowance plus travel to and from work: Barnardo's: Purpose ...

Recruitment Genius: Content Writer - Global Financial Services

£25000 - £30000 per annum: Recruitment Genius: From modest beginnings the comp...

Recruitment Genius: Web Developer - PHP

£35000 - £40000 per annum: Recruitment Genius: From modest beginnings the comp...

Recruitment Genius: Customer Service / Receptionist

£15000 - £24000 per annum: Recruitment Genius: This is an exciting opportunity...

Day In a Page

The Silk Roads that trace civilisation: Long before the West rose to power, Asian pathways were connecting peoples and places

The Silk Roads that trace civilisation

Long before the West rose to power, Asian pathways were connecting peoples and places
House of Lords: Outcry as donors, fixers and MPs caught up in expenses scandal are ennobled

The honours that shame Britain

Outcry as donors, fixers and MPs caught up in expenses scandal are ennobled
When it comes to street harassment, we need to talk about race

'When it comes to street harassment, we need to talk about race'

Why are black men living the stereotypes and why are we letting them get away with it?
International Tap Festival: Forget Fred Astaire and Ginger Rogers - this dancing is improvised, spontaneous and rhythmic

International Tap Festival comes to the UK

Forget Fred Astaire and Ginger Rogers - this dancing is improvised, spontaneous and rhythmic
War with Isis: Is Turkey's buffer zone in Syria a matter of self-defence – or just anti-Kurd?

Turkey's buffer zone in Syria: self-defence – or just anti-Kurd?

Ankara accused of exacerbating racial division by allowing Turkmen minority to cross the border
Doris Lessing: Acclaimed novelist was kept under MI5 observation for 18 years, newly released papers show

'A subversive brothel keeper and Communist'

Acclaimed novelist Doris Lessing was kept under MI5 observation for 18 years, newly released papers show
Big Blue Live: BBC's Springwatch offshoot swaps back gardens for California's Monterey Bay

BBC heads to the Californian coast

The Big Blue Live crew is preparing for the first of three episodes on Sunday night, filming from boats, planes and an aquarium studio
Austin Bidwell: The Victorian fraudster who shook the Bank of England with the most daring forgery the world had known

Victorian fraudster who shook the Bank of England

Conman Austin Bidwell. was a heartless cad who carried out the most daring forgery the world had known
Car hacking scandal: Security designed to stop thieves hot-wiring almost every modern motor has been cracked

Car hacking scandal

Security designed to stop thieves hot-wiring almost every modern motor has been cracked
10 best placemats

Take your seat: 10 best placemats

Protect your table and dine in style with a bold new accessory
Ashes 2015: Alastair Cook not the only one to be caught in The Oval mindwarp

Cook not the only one to be caught in The Oval mindwarp

Aussie skipper Michael Clarke was lured into believing that what we witnessed at Edgbaston and Trent Bridge would continue in London, says Kevin Garside
Can Rafael Benitez get the best out of Gareth Bale at Real Madrid?

Can Benitez get the best out of Bale?

Back at the club he watched as a boy, the pressure is on Benitez to find a winning blend from Real's multiple talents. As La Liga begins, Pete Jenson asks if it will be enough to stop Barcelona
Athletics World Championships 2015: Beijing witnesses new stage in the Jessica Ennis-Hill and Katarina Johnson-Thompson heptathlon rivalry

Beijing witnesses new stage in the Jess and Kat rivalry

The last time the two British heptathletes competed, Ennis-Hill was on the way to Olympic gold and Johnson-Thompson was just a promising teenager. But a lot has happened in the following three years
Jeremy Corbyn: Joining a shrewd operator desperate for power as he visits the North East

Jeremy Corbyn interview: A shrewd operator desperate for power

His radical anti-austerity agenda has caught the imagination of the left and politically disaffected and set a staid Labour leadership election alight
Isis executes Palmyra antiquities chief: Defender of ancient city's past was killed for protecting its future

Isis executes Palmyra antiquities chief

Robert Fisk on the defender of the ancient city's past who was killed for protecting its future