North of Rome, in Umbria, a series of ancient towns perch on the tops or sides of the foothills. Tourists flock here to admire the medieval churches and palazzos built with the unlimited local supplies of limestone. But no one gives any thought to the creatures that helped to create the materials necessary for making such stunning, long-lived monuments.

Limestone is composed largely of crystallized calcium carbonate. Some of it comes from the skeletal remains of well-known creatures like corals, but much of the rest comes from less appreciated but truly remarkable organisms called foraminifera, or forams.

Forams have been called "nature's masons" and deservedly so. Most of the 6,000 species of these single-celled protists construct surprisingly complex, ornate and beautiful shells to protect their bodies.

A broader pattern revealed

After forams die, their shells settle in ocean sediments -- and may eventually become rocks that can be used to shelter our bodies. While tiny relative to ourselves, forams are extremely large for single-celled organisms. The most impressive of all forams, now extinct, were disk-shaped species that were abundant in the limestone used to build the great pyramids of Egypt.

In more recent times, forams have served an altogether different service -- to science. Because they produce shells that make good fossils, and have long been abundant and widespread in the oceans, forams are particularly valuable to geologists and paleontologists in telling us about Earth's history.

The forams in the limestone just outside Gubbio provided the first clues to one of the most exciting scientific discoveries in the past century.

In the 1970s, geologist Walter Alvarez was studying the limestone formations. Because different species with different shell shapes evolved at different times, foram fossils have been used to date rocks. He learned that the topmost layer of the rocks from the Cretaceous period -- the end of which coincided with the extinction of the dinosaurs -- contained a diverse array of large fossil forams. But the rock layer above it, which signaled the beginning of the Tertiary period, contained only a few, much smaller species. Separating the two rock layers was a layer of clay that appeared to lack fossils altogether.

The geologist Jan Smit discovered a similar pattern in southern Spain.

What could have caused the disappearance of widespread, tiny organisms as well as much larger creatures? As it turned out, it wasn't something on Earth but something from space. Chemical analyses of the clay layer revealed that it contained extraordinary levels of the element iridium, a material rare on Earth but more abundant in certain kinds of asteroids. The scientists proposed that the iridium was fallout from an asteroid that hit Earth at the end of the Cretaceous period, 65 million years ago.

Warning of a new threat

The asteroid was about the size of Mount Everest and hit Earth at about 50,000 mph, drilling a 120-mile-wide crater underneath Chicxulub, Mexico. It ejected so much material into (and even out of) the atmosphere that food chains were disrupted for thousands of years. The impact caused one of the greatest mass extinctions in history.

Eventually, forams and the oceans rebounded, and new species evolved.

Today, forams are warning us of a new threat, for they are not merely witnesses to history, but participants in it. Forams are a vital part of a "biological pump" that removes carbon dioxide from the atmosphere. The increasing levels of carbon dioxide in our atmosphere, now at a greater level than at any time in the past 400,000 years, threaten to overwhelm this pump. Ocean acidification may be a sort of stealth asteroid of environmental change with potentially catastrophic effects on food chains. And in the current period, guess who is at the top of those chains?