Tag Archives: evolution

De rerum…corpus individuum? On the nature of the periodic table.

Through the caprices of classroom scheduling, I find myself teaching a writing class in a seminar room usually devoted to the study of the grammar of electrons and the narratives of covalent bonds. My colleagues in the Chemistry Department have posted a periodic table on the wall at the front of the classroom. This arrangement is typical. You’ve likely sat in such a room. Science classrooms, especially lecture halls, often have the Table lifted in front of and above the pews, in the same location as the stained glass window or crucifix above a church’s high altar.

There is a message here that extends beyond the practical utility of having a convenient chart of the elements at hand. Behold, congregation: This is the nature of our Universe. Order exists beyond the confusion of form. And indeed, this is the nature of things (thank you, Lucretius), if we restrict our gaze (as we must when we teach any class) to one particular scale of our world (about 0.3 nanometers, atom size). But the Table’s lofty and lonely position — it is seldom accompanied by any other words or symbols — is also misleading. The world is ranged in rows and columns only in one very particular way. For a biologist or a subatomic physicist, other images are more appropriate. Years ago, when I taught Introductory Biology to a large lecture-hall of first-year students, I would lower the projection screen to veil the Table, replacing it with images of ecological communities or individual living creatures. My purpose was not to deny the power or utility of the atomistic view, still less to claim that the study of chemistry is not relevant to biology, but to reclaim visual space within our science classrooms for the messy beauty of life: unpredictable, diverse, braided.

From Darwin, at the close of The Origin (I quote from the first edition):

It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us…whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

Out of physical order, an unfolding process, one that cannot be contained within ruled lines.

In every classroom from whose walls the Table gazes, let us place an equally-sized artwork of the local community of life, humans included. A twin for what Whitman called “the figures…ranged in columns before me.” An entangled bank interpreted through art and science, the “endless forms most beautiful and most wonderful” of the human mind. That, or a classroom window.

Mammal fest

We’ll get to weighty matters shortly, but first some photos of gratuitously cute mammals from my trip to Colorado. One of the more zoologically startling features of the western U. S. is the number of mammals wandering in plain view. The furry-faced, doe-eyed, jaunty-eared denizens of Tennessee tend to stay out of sight in the thick woodland, so mammalogists must be content with scat and sketchy tracks (not counting the white-tailed deer on the porch). Not so in the wilder parts of Colorado.

Golden-mantled ground squirrel. Mueller State Park, Colorado.

Golden-mantled ground squirrel. Mueller State Park, Colorado.

Richardson's Ground Squirrel. Florrisant Fossil Beds National Monument.

Richardson’s Ground Squirrel. Florissant Fossil Beds National Monument.

Colorado Chipmunk. Florrisant Fossil Beds National Monument.

Colorado Chipmunk. Florissant Fossil Beds National Monument.

Mule Deer. Mueller State Park.

Mule Deer. Mueller State Park.

Bobcat. Florrisant Fossil Beds National Monument.

Bobcat. Florissant Fossil Beds National Monument.

Trespass being considered by Cottontail rabbit (either Mountain or Eastern Cottontail, the bobcat would know for sure). Florrisant Fossil Beds National Monument.

Trespass being considered by Cottontail rabbit (either Mountain or Eastern Cottontail, the bobcat would know for sure). Florissant Fossil Beds National Monument.

After a few days wandering among these animals and flipping through the field guide, a question emerges: Why so many squirrely creatures in the west? The Field Guide to the Mammals has twenty four pages of ground squirrels, almost all from the west. The one eastern species, the eastern chipmunk, has twenty two cousin species in the west. A rodent biodiversity bomb has flung fat-cheeked squeakers into every grassy glade, forest edge and rock pile west of the Mississippi.

A recent paper by Ge et al, evolutionary biologists working at the Chinese Academy of Sciences, helps us to understand what happened. The ground squirrels (Xerinae, a subfamily comprising the chipmunk, marmots and other ground squirrels within the rodent family, the Sciuridae) originated in Europe about thirty five million years ago. From there the animals spread to Africa and North America. The group’s explosive diversification happened later, about sixteen million years ago, and seems to have been triggered by climatic changes.

The same cooling drying climate that separated the American and Asian forests created a grand opportunity for ground squirrels. As forests retreated, open meadows, grassy plains and sparsely vegetated mountainsides took over, especially in the arid west. This change, combined with the highly variegated landscape (not too many other places in the world have such a complex mix of mountains, plains, deserts), caused the ground squirrels to speciate into the many species we see today. Some of these then colonized other parts of the world, probably after they moved west over the Bering land bridge. For the marmots, chipmunks and ground squirrels, Manifest Destiny did not end at the Pacific Ocean. They kept going and colonized Siberia, Asia and Europe: the far, far, Wild West.

We hear in this story echoes of our own. Homo sapiens is also a creature of the grasslands, brought into being by forests in retreat from a cool, dry climate.

Note how the ground squirrels survey their homes: on two legs.

Note their social systems: they often live in close-knit groups in stable villages.

We’re lucky that their thumbs are not quite as agile as our own and that their food didn’t take quite as much brainwork to catch.