Conifers are a group of seed plants (taxonomically an order, subclass, class, or division), all of which are descended from a common ancestor in the late Paleozoic, more than 300 million years ago, that they do not share with any of the other four living groups of seed plants. The nearly 550 species of conifers are found all around the world (although in differential abundance and prominence), on every continent (except Antarctica), and on many islands. Many conifers are familiar plants, especially those belonging to the most widespread genera: pines (Pinus), firs (Abies), spruces (Picea), and junipers (Juniperus) in the northern hemisphere (Plate 1), and yellowwoods (Podocarpus) in the southern. Taken together, these five genera contain about 300 species, more than half the living conifer species, and occur in almost all the places where any conifers are found. All but about 15 species of conifers are evergreen, even in temperate and colder climates. Most flowering plants are also evergreen (especially those of the tropics and warm temperate regions) but typically are referred to as broad-leaved evergreens to distinguish them from the needle- and scale-leaved conifers. While the majority of conifers have needle-, scale-, or clawlike leaves, a few species have broader leaves that are a far cry from pine needles or juniper scales. Despite some variations, however, their distinctive leaf forms are among the most obvious characteristics uniting the conifers, since most of these forms are shared across the different families.