Geology Reference
In-Depth Information
Fast-forwardto65millionyearsagoandoneoftheworstdaysinEarthhistory.Anaster-
oidestimatedtohavebeenaboutsixmilesindiametercollidedwithEarthnearwhatisnow
the Yucatán Peninsula. An epic mega-tsunami swept the globe, followed by massive fires
that burned across entire continents. Immense clouds of vaporized rock darkened the skies
and all but shut down photosynthesis. This cosmic trauma appears to have descended on a
worldalreadyatrisk.Inanechooftheend-Paleozoicextinctionevent,agreatseriesofvol-
canic eruptions in India may have already been altering Earth'satmosphere and weakening
its ecosystems for hundreds of thousands of years. In another echo, a significant sea level
dropappearstohaveexposedmuchofthecontinentalshelfataboutthattime,upsettingthe
ocean's food web and killing off all but eight known ammonite species out of thousands.
Reasons for such a sea level change are not at all obvious for there was no ice age at the
time.Somescientistsspeculatethatmidoceanridgesbecamelessactive,causingacooling,
contraction, and consequent sinking of the entire ocean floor.
Whatever the causes, individually or in concert, all the dinosaurs except for one minor
lineage—the birds—went extinct. The last of the ammonites also died out. The way for
evolving mammals was paved. These small, rodentlike vertebrates had become well es-
tablished in the company of their larger (and therefore doomed) dinosaur brethren, and
their survival of the end-Mesozoic extinction gave them footing in almost every ecological
niche. Within ten million years of the Indian megavolcano and coincident asteroid impact,
mammals had diversified; within fifteen million, early ancestors of whales, bats, horses,
and elephants had evolved.
So it was that mass extinctions repeatedly challenged and winnowed life on Earth. The
last 540 million years have seen this ebb and flow again and again. But what of earlier
times?Weretherenomassextinctions priorto540millionyearsago?Herepaleontologists
are stumped. Prior to the Cambrian explosion, there are almost no diagnostic fossils to re-
cord. The statistics of extinction require significant numbers of distinctive organisms like
dinosaurs and trilobites; before 540 million years ago, they simply don't exist. We can be
almost positive that microbial life went through periods of trauma and species loss. There
must have been giant asteroid impacts andepisodes ofdestructive volcanism that sterilized
significant fractions of Earth's surface. Certainly life was severely challenged during the
snowball Earth episodes, perhaps during earlier glaciations as well. There could have been
hundreds of mass extinctions stretching back to the very dawn of life. But we may never
know from the spotty, microscopic Precambrian fossil record.
The Human Age
For more than 99.9 percent of Earth's existence, there were no humans. We are but an
eyeblink in our planet's history.
