Sorry, but are we related?

The Variety of Life by Colin Tudge (Oxford University Press, £35) Deep Time by Henry Gee (Fourth Estate, £20)

"O wonder!", cries Miranda in The Tempest, "How many goodly creatures are there here!". Shakespeare was spot on as usual. A simple list of every creature now known to live on Earth would fill seven fat volumes, Colin Tudge calculates. It could run to 70 or more were we to include the living species yet to be discovered. "And if we could find out all the creatures that have lived in the past we would need a substantial library - just to make the barest list," he writes. "How can we possibly cope? Classify, is the answer."

Taxonomy may sound dull, but Tudge makes a brilliant case for it. A seasoned science writer with a delightfully light touch, he can make the most arcane subject appealing. But The Variety of Life is far more than just another good popular science book. It's a celebration of the "huge privilege" it is to share the planet with so many fantastical creatures.

Tudge has clearly been plagued by well-meaning friends who endlessly enquire, "who is this book for?". With characteristic doggedness, he has carried on regardless. The result is a beautifully illustrated introduction to everything that's ever lived. It bears the hallmarks of genuine enthusiasm, as opposed to the dead hand of commercial opportunism.

Tudge spent a decade on the book, with a mission in mind: to bring taxonomy, or the science of systematics, back to the centre of biological teaching and thinking. Classification is not "a dull pursuit for obsessives," he argues. Rather, it is a vital guide to the world's bewildering biodiversity. We need to know what's out there, if only to exploit and control it, but especially to appreciate and conserve it.

At the heart of Tudge's romp through the living world is cladistics, the methods of classifying organisms developed by German entomologist Willi Hennig in the 1950s and 1960s. With the help of an analogy about four chaps in a pub, Tudge presents an accessible introduction to this notoriously difficult classificatory tool.

He delicately alludes to the controversy in the early 1980s, when the supporters of cladistics at the American Museum of Natural History and the Natural History Museum in London were accused of "Marxist subversion". Cladists were "Mad, Bad and Dangerous to Know," remembers Henry Gee, now a senior editor at Nature. "To an impressionable student like me, this combination exerted a magnetic attraction."

Just why an obscure method of classifying beetles or bandicoots should have aroused such fury is the theme of Gee's entertaining Deep Time. Died-in-the-wool traditionalists were right to be suspicious of cladistics, Gee argues, for the approach has radical implications of our understanding of evolution: "Cladistics represents a revolution in thought as profound as that of Darwinian evolution by natural selection".

Cladistics has turned palaeontology "from a book of children's stories into a true science," Gee asserts. Consigned to the dustbin are all those just-so-stories about why the dinosaurs went extinct, say, or how birds first took to the air. Such causal narratives are unscientific because they can never be tested, Gee argues. We need to realise that the "fossil record" is nothing of the kind. All we have to go on are scattered remains preserved by chance: a few fossils that stand as "isolated tableaux illuminating the measureless corridor of deep time".

The immensity of the geological time-scale over which life has evolved makes a nonsense of any attempt to construct scientific stories populated by ancestors, descendants and "missing links": "Deep Time is not a movie but a box of miscellaneous, unlabelled snapshots." Forget "meet the ancestors"; the most palaeontologists can aspire to is to discern degrees of relatedness, for all organisms are our cousins, close or distant.

This insight forms the intellectual bedrock of cladistics, the classificatory scheme that has now become the orthodoxy among systematists. The goal is to create "cladograms" - branching diagrams that connect living things by their shared characteristics. Cladograms are not family trees, because they make no assertions of direct descent. Meeting the ancestors is acknowledged to be an impossible dream.

Gee says this book "took much longer to write than I had imagined" and signs of the struggle seem just discernible. Some chapters whiz along, notably "The Gang of Four", where Gee describes the summer studentship he spent ensconced in the fossil fish section of the Natural History Museum, which turned out to be the intellectual headquarters of cladistics in Britain. His account of lunchtimes spent with the gang in a seedy pub on the Old Brompton Road is a classic piece of reportage.

He is excellent too at conveying the buzz that scientists feel every time a piece of original research knocks a dusty old textbook account off its perch. Gee says cladistics fosters such fresh thinking because it casts aside traditional preconceptions about functions or adaptation. So cladistically-minded researchers working on an early tetrapod fossil noticed that the thing has eight digits, when previous workers stopped looking when they reached the expected five. Colleagues found fossil dinosaurs sprouting feathers that had nothing to do with flying.

There are times when Gee seems to overstate his case. You want to ask the forbidden question, "If feathers did not evolve for flight, what did they evolve for?" Gee is stern: "As we now know, such questions about adaptive purpose are unanswerable."

As he reiterates, the living creatures fossils once were are long gone, so we can never know much about what life was like for them. Still, isn't it interesting, even legitimate, to speculate that the feather-like things on some dinosaurs could have provided thermal insulation or been used for sexual display? Can palaeontology do without such imaginings?

A few years ago, even Nature called a new human-like fossil a "missing link" in its press release. At the time, writes a contrite Gee, it "seemed a more digestible substitute for phrases such as 'the hominid-closest-to-the-evolutionary-split-between-our-lineage-and-that-of the-apes'." The Daily Express gave the story upbeat coverage. It recast the fossil as Fred Flintstone's honoured ancestor, under the headline "Yabba Dabba Doo!" It's no contest when cladistics meets the comics. But both Tudge and Gee deserve high praise for these stimulating additions to the popular-science canon.