In 1829, the gathered members of Australia’s first scientific society selected as their motto: “Quocunque aspicias hic paradoxus erit.” Or, “Whatever one examines here will seem a paradox.”

In his new book “Chasing Kangaroos: A Continent, a Scientist, and a Search for the World’s Most Extraordinary Creature” (Grove Press, 248 pages, $24), zoologist Tim Flannery brushes aside this statement as an inadequate, misguided kind of Australian biological exceptionalism. The phrase, he writes, “well captured the received British perception that everything in Australia was, contra naturam, novel to the point of being ridiculous.”

Mr. Flannery seeks to demystify the animals he calls “the most remarkable animals that ever lived, and the truest expression of my country.” But he is not immune to a certain exuberant awe himself. He charts the evolution of marsupials as they branch off into wallabies and rat kangaroos and beyond, and then he follows those steps backward in a hunt for “the first kangaroo.” Somewhere in between, he traces his own evolution as a scientist. It began when, after a spectacular failure in Zoology 101, Mr. Flannery met the California paleontologist Thomas Rich. Mr. Rich granted him the privilege of cleaning kangaroo fossils in the bowels of the Museum of Victoria, and Mr. Flannery took to it immediately, happy to eat beans for lunch and talk about bones. Mr. Rich, a lifelong friend and mentor, provides what becomes Mr. Flannery’s own motto: If you want to study the history of this country, you’ll have to have the will to fail.

In “Chasing Kangaroos,” Mr. Flannery’s concern for the continent’s Aboriginal population also emerges. Last month, the Australian Parliament approved new laws that enact strict, some say paternalistic, oversight over Aborigines, who constitute 2.7% of the national population. The regulations include the banning of alcohol in areas in the Northern Territory, and Mr. Flannery’s visits to many indigenous communities bear out the impetus behind the legislation. During a stop at Groote Eylandt to study the agile wallaby and rockwallaby, he meets the leader of the island’s Aboriginal community, Claude Mamarika, whose son was killed driving drunk just weeks before. Alcohol, unemployment, and the resulting violent crime have ravaged the island. In an earlier visit to a rundown, remote encampment, Mr. Flannery notes that the clicking of beer cans has replaced traditional clapsticks in the Aborigine ceremony called the corroboree.

Mr. Flannery is not afraid to get his hands in the muck, all in his quest for fossils, live specimens, and local lore. He uses a “defleshing knife” to saw the head off a dead man-size ‘roo he finds by the side of the road; traverses moist ground covered in scorpions; and kisses the Opal Queen of Coober Pedy, “a lady of generous proportions,” resplendent in fishnets and a miniskirt.

Luckily for the author, his hard times often pay off: The natural history writer Redmond O’Hanlon has said of him, “He’s discovered more new species than Charles Darwin.” At one point, Mr. Flannery spends days shuffling on all fours in the outback, scouring the ground for the missing evolutionary link between kangaroos and their possum-like ancestors, only to find a square bone fragment the size of a match head that he quickly identifies as the anklebone of “the grandfather of all kangaroos.”

In his introduction, Mr. Flannery compares his book to putting together an incomplete jigsaw puzzle. He often flashes back to a motorcycle trip he took as a 19-year-old, attempting to circumnavigate the continent. The journey adds narrative thrust, but, interspersed with tales from Mr. Flannery’s other expeditions, centuries of Australian cultural and biological history, and very occasionally dry scientific passages, it can be difficult to remember what decade the reader is in—and this in a book primarily concerned with rigorously pinning down the marsupials’ own timeline. But, then again, what’s a decade among millennia?

“Chasing Kangaroos” is a warmhearted book by an expert and an enthusiast for his subject. “So breathtakingly different is the kangaroo that if it did not exist we’d be unable to imagine it,” Mr. Flannery rhapsodizes. “They are, in my opinion, the most remarkable animals that ever lived.” It’s a function of this boundless awe that he’s still in love with the damn things. Whatever we do not tire of must retain some spark of unpredictability, some otherness, to keep us enthralled. Mr. Flannery’s fervor for the animals of the fifth continent has kept him curious for three decades. That’s 22 years longer than the average Australian marriage.

Ms. Graham is an editor at Domino magazine.