“Once, years ago, I left home looking for a grand adventure and spent five months staring at mud.”

Robert Moor introduces himself and his subject in “On Trails” with that nearly perfect invitation: The words flow effortlessly toward an optimistic ambition (with a touch of irony in that “grand”) and end with the bump of unadorned reality, which you know even then will be metaphysically transformed before you can say, “Appalachian Trail.”

Hiking the AT from Springer Mountain in Georgia to Mount Katahdin is the adventure Moor is referring to. He had the misfortune to pick the summer of 2009, which he says was so awful it was compared to the “year without a summer” in 1816.

“My memories … consist chiefly of wet stone and black earth,” he writes. He had anticipated months of looking up and out, the trail a mere vehicle for seeing what was all around him. Looking down instead, all he could do was study the trail as an actual thing. Which led to questions: What is a trail really? Does it go forward or backward? Who made it and why?

The result is a wonderfully rich and human book. It is a trail all on its own, marked by the procession of internal contemplation and idea-spinning that a long solitary walk in the woods can produce.

Moor is interested in everything, with a knack for communicating that curiosity to the reader, and he has lined up a chorus line of experts that are as one-of-a-kind as their various specialties.

The book’s one organizing principle is evolutionary chronology. Moor starts off with fossil tracks so old that scientists can’t decide whether the organism that made them was a plant or an animal or something else altogether. This Ediacaran trail-blazing may have been nothing more than an effort to stay put. Involuntarily displaced by wind or wave, it was just struggling “through the muck to regain its perch.”

One can only guess about what really motivated the Ediacarans. Within the modern insect world, scientists continue to gain amazing insights into the ways insects use trails, laying them down with chemical pheromones, to communicate where (and where not) to find food and how much there is. Moor finds plenty of contemporary researchers to interview, but he is equally fascinating on the history of these discoveries and the discoverers themselves.

The author divides interactions and relationships between higher animals and humans into watching, herding and hunting. As he investigates these three paradigms – “Naturally, I began with the one that intimidated me least,” he admits with delightful self-deprecation – he follows a caregiver at a retirement home for circus elephants (someone he first met hiking the Appalachian Trail); works as a rather inept shepherd on a Navajo farm; and hunts with an Alabaman bowhunter (unsuccessfully – one feels that Moor was relieved, although he gives a graphic description of what might have been).

When it comes to human trails and path-finding, he is taken all over Cherokee country by an unlikely environmental activist (a former member of the John Birch Society), is instructed in trail craft in the jungles of Borneo, and hikes part of the Appalachian Trail with the only full-blooded Cherokee to complete a thru-hike.

After that, Moor discovers the International Appalachian Trail, the creation of former Maine Conservation Commissioner Dick Anderson. He joins Anderson at a conference in Iceland, the island country a symbol of the parting of the continents along whose rim the IAT follows the ancient traces of the Appalachian Orogeny. He ends up in Morocco hiking the last stretch of what he calls “the world’s longest hiking trail.”

“On Trails” covers a lot of ground. Moor has a wonderful sense for the original, vivid metaphor or description. On a field of larger fossils, the Ediacaran trails are “like a poem carved onto a handrail in a stairway of the Louvre.” An approaching storm cloud “let out a soft digestive growl.” Fascinating facts fall fast and furiously: algorithms from ant colony pathways have improved British telecommunications networks; slime molds have independently come up with a network identical to the Japanese rail system in Tokyo. “Linger a moment over that fact,” he writes, gob-smacked. “A single-celled organism can design a railway system just as adroitly as Japan’s top engineers.”

Moor writes that his aim was to create a “trail winding from the dim horizon of the past to the wide foreground of our present circumstances.” He has succeeded admirably. Thru-hikers be warned: you’ll be ditching some essentials to make room for “On Trails” in your pack.

Thomas Urquhart is a former director of Maine Audubon and the author of “For the Beauty of the Earth.”