Central America is one of the worlds connecting points. The land bridge that now exists through Panama (though one that even today is not open to vehicles) is – in geological terms – brand new. It was created only about five million years ago. Few such bridges currently exist (although many have been built and destroyed in earlier times). Panama is unquestionably now the most remarkable in evolutionary terms, since others have either been very important in the past but are now beneath the oceans (like the Bering Strait) or are significant geographically (Sinai) but do not provide a bridge for flora and fauna in the unique way that Central America does.

It is because it is a bridge that Central America is so fantastically diverse in its wildlife. In the small national park next to where I live in Nicaragua, there are over 180 species of butterfly (the whole of the British Isles accounts for 59 species). Central America is a meeting and crossover point for species, not just the obviously mobile ones like the mammals, but for trees and shrubs too. This is what David Rains Wallace brings to life in the explorations of different parts of the link between continents that make up his fascinating book.

When South America separated from Africa and the Atlantic Ocean was formed, it took with it animals whose ancestry can be traced back to their African cousins. What it didn’t take were big cats. This was a factor in the emergence of huge mammals like ground sloths in South America, unaware until about two million years ago (when the interchange started in earnest) that they were about to be wiped out by saber tooth tigers due to arrive from the north. On our farm we have pochote trees which have spiny trunks. They provide no obvious defence against current wildlife, but they may well have evolved to defend trees against the gigantic herbivores of the past.

Central America is both a testament to the interchange and a continuing part of it. It has a legacy of wildlife which is an exotic mixture of animals from north and south. Squirrels from the north share the trees with monkeys and the occasional sloth from the south. Improbably, iguanas seem to have moved south into Central and South America, then to have largely died out in the North. Bird species like the enormous variety of humming birds (unique to the Americas) moved from south to north, as did tree species like the Guapinol (one of the oldest tree species known). Conversely, pines extend south from North America right into Nicaragua, where their spread comes to a halt. In fact, our neighbours not long ago cut down a Scots pine-like tree: it may have been the most southerly representative of its species in the continent. Pines reappear of course in South America, but of different species.

Rains Wallace complains that for a region so rich in wildlife there is a remarkably small literature about it. Since he wrote the book in the late 1990s, this has changed. Last week we had a visit from Fiona Reid, whose guide to the Mammals of Central Amercia and South East Mexico appeared a decade after the Rains Wallace book. Just flicking through the illustrations in her book (which she drew herself) is to move through a gallery of travellers from south to north and vice versa – spider monkeys from the south, jaguars from the north, anteaters from the south, squirrels from the north and so on.

Every living thing that surrounds us here in Masaya either reflects this ancient interchange or the more modern one that began when Columbus arrived on mainland Central America 500 years ago, that led to the import of bananas, mangos, coffee, rice and other modern staples, and the export of beans, maize, yuca (cassava) and many more. Even my wife’s ancestors arrived from the North (and have left axe-heads in our farm). My own presence, shamefully, owes more to Columbus.