"All in good time, ladies and gentlemen, please. All in good time. First, Doctor Golgachev, our paleohistorian, has some interesting facts concerning the larger picture. I would like to introduce him at this time."
An imposing man with bushy eyebrows and a stern face, his voice was nonetheless disarmingly cordial and engaging. His large hands resting on either side of the podium, he casually scanned the roomful of reporters. Then, he began: "I would first like to thank Doctor Tolstoy for his leadership during this time. For, what we have discovered thus far has been, ..., deeply disturbing and profound." He turned to nod towards Doctor Tolstoy, who nodded back. We were waiting; but not for long, he started right in.
"My name is Alexi Golgachev; I am a paleohistorian -- the history of old Earth. Doctor Tolstoy has requested that I present a brief overview of background material relating to Earth events leading up to and immediately following -- geologically speaking -- when the alien structure evidently either first or last made contact where it now rests. Major geologic and biologic transformations occurred during this time period. A whole new order of biology emerged simultaneously all over the world. One Brink after another was crossed as evolution shifted gears, as life discovered original ways to express itself, ways that were previously unknown. No one just considering organic life prior to 600 million years ago -- the time the alien craft is determined to have landed -- would or could conjecture the direction life would take afterwards.
"My lecture will be brief, so please bear with me."
Sipping from his glass, he stared off into the distance, his eyes glazing over momentarily, then refocusing with a fierce intensity. "It is generally accepted that the Earth is four point five billion years old. Formed by the collision of millions of asteroids, comets, meteors, and other debris of the solar system, it was a ball of molten rock whose temperature was akin to that of the sun's -- 8,000 degrees fahrenheit. But, from the very beginning, it began to cool; its surface eventually forming a hard crust covering the entire Earth. It was a strikingly barren, waterless wasteland, completely devoid of life.
"Bombardment by meteors is generally accepted as the source of water for the Earth. Scientists are divided on this, however. But, in any event, water quickly evaporated to join the other constituents of the primeval atmosphere, blocking the sun's light. Around four billion years ago, it started to rain. And rain it did, for millions of years, producing the single vast ocean. The Earth's temperature at this time had cooled to a pleasant 200 degrees fahrenheit.
"As early as approximately 3.8 billion years ago, life began in the form of simple, unicellular microbes. The Archaea, the most ancient of life-forms, capable of living in the harshest of environments, are not direct ancestors of, but were followed in time by, the prokaryotic cell arrangement -- photosynthetic bacteria -- and eukaryotic -- those with a nucleus containing the chromosomes. Back then, the primordial atmosphere was extremely poisonous, consisting of methane, ammonia, molecular hydrogen, copious amounts of carbon dioxide -- the result of severe volcanism during this time -- and water vapor. As well, the Earth was continuously bathed in deadly ultraviolet light.
"But, in spite of these conditions, three and a half billion years ago, stromatolites -- basically layered mats of prokaryotic cyanobacteria, laminated accretionary structures -- were busy exchanging carbon for oxygen in the atmosphere. Eukaryotic algal stromatolites didn't appear until a good two billion years after these communities of microorganisms began forming. These terraforming creatures had their work cut out for them, and, fortunately, they were very successful, not to mention extremely tough and resilient. During this same period, carbon dioxide dissolved in seawater combined with calcium to form limestone deposits on the bottom of the seas. This methodical, steady process went on for two billion years.
"The Archaean oceans were olive-green due to iron released from Earth's interior as well as from rivers and streams running down to the seas, but because of the thick blanket of carbon dioxide in the atmosphere, they would have appeared red, had there been anyone to see them. As quickly as it was produced, free oxygen was gobbled up by this raw iron to form banded iron formations [BIFs] until around 2.1 to 2 billion years ago. With the bedding of iron oxide, there was a dramatic increase in the percentage of oxygen in the atmospheric-oceanic system. The oceans turned from green to blue and the atmosphere cleared for the first time in Earth's history. Possibly as long ago as two billion years, certainly 1.8 billion, due, largely to the stable oxygen-rich environment, aerobic eukaryotes, of the type that make up plants, fungi, and animals, came into being, or invented themselves, through a symbiotic internalizing of specific one-celled bacteria which transformed into mitochondria or chloroplasts, depending.
"The early forms reproduced by mitosis, or self-cloning, the very same process that our cells and those of all plants and animals utilize to replicate. This is similar to the cell-fission of prokaryotes of a single, non-nucleated chromosome in which the single chromosome in a parent cell is duplicated and passed on to daughter cells as the parent splits in half, but far more complicated than simple cloning, having to first break down the nucleated sac.
"Evolution for the eukaryotes began in earnest with the advent of sexuality, the reproductive process called meiosis -- sex cell division. It is believed this occurred about 1.1 billion years ago. Large eukaryotes, programmed for development, a major breakthrough, arose near the end of the Precambrian.
"After some critical point had been reached in the concentration of oxygen, tiny, multicelled tubular worms and other invertebrates came on the scene, burying carbon as fecal pellets in the sediments beneath the seas, at least as far back as 800 million years. This accelerated a net loss of carbon in the system accompanied by a steady rainfall of limestone depositions.
"Furthermore, during the period from about 900 million years ago to 600 million, after a rapid increase, there was a gradual decline and die-off of the world's unicellular eukaryotes, in particular, large-celled microalgal flora. An exponential increase in oxygen production and carbon burying, resulting in a decrease in carbon dioxide production, are generally accepted as being responsible. In addition, the photosynthesizing capability among these creatures was blocked by the failure of a crucial enzyme to be produced.
"Why, exactly, is unknown.
"Furthermore, the non-photosynthesizing heterotrophs, having to rely on an external carbon source, either perished in the dearth or were greatly reduced. Prokaryotic cyanobacteria, however, possessed of their own internal carbon source, sailed through this extinction barrier.
"This event precedes, barely, the advent of the programmed eukaryote."
He paused to consider the bewildered faces before him. Then, with a slight smile, he relaxed further into his material, his voice deepening -- the professor lecturing to a freshman class. He continued: "This geologic time period -- from Earth's formation to about half a billion years ago -- is called the Precambrian Eon; the present eon being the Phanerozoic. Specifically, we are concerned with the latter part of the Precambrian, called the Proterozoic Era, which dates from 2.5 billion to 550 million years ago.
"During that same period, the Earth suffered through at least four major ice-ages brought on by increased oxygen levels and a corresponding decrease in carbon dioxide. The last being the Varangian glaciation, approximately 610 to 590 million years ago, covering all of the Earth -- "Snowball Earth" -- and lasting many millions of years. Extreme volcanism, belching a blanket of warming carbon dioxide, is thought to have brought an end to this near catastrophe for life.
"Close on the heels of this event, a large and diverse assemblage of distinctly different kinds of worm-like and jellyfish-like creatures suddenly emerged; the oldest many-celled animals -- the Ediacaran Fauna. They were first suspected of having made their appearance as early as 800 to 700 million years ago -- the beginnings of many-celled animals -- later evolving in size and diversity on a scale that had not previously existed. But closer scrutiny has revealed that these initial creatures actually resemble colonies of protozoans, linked in some kind of endosymbiotic fashion -- layered aerobic/anaerobic mats. Whereas, the creatures of the Ediacaran possessed a genuine cellular diversity, albeit narrow in scope, with an accompanying integration of functions. They were not bony creatures -- this is an important point -- they used water pressure and in some cases sand to give themselves a semblance of rigidity -- called turgidity. For that reason, our fossil record is one of organic imprints in soft clay, sandstones and mudstones only."
Golgachev slowly scanned the hushed and expectant audience, then continued: "Just prior to the Cambrian Period, the Ediacaran Fauna, as an integrated ecosystem, collapsed and disappeared, only a few of the more common phyla passing through the extinction barrier. Also, it should be pointed out, in rock dated about 10 million years earlier, small shelly fossils, microscopic phytoplankton for the most part -- foraminifera, marine protozoans, for example -- suddenly appeared, and, in the seas under the ice of the Verangian, settled the Earth within a few million years.
"The White Cliffs of Dover are composed of such chalk. These shelly creatures characterize the base of the Cambrian."
He sipped water, wetting his lips for the finale: "All the bodyplans for all living animal species currently existent -- all the hard-bodied animal phyla -- those capable of leaving skeletal fossils, abruptly came into being around 545 million years ago, at the onset of the Cambrian Period, and either spread throughout the world in a brief span of time, or arose simultaneously everywhere.
"That's why it's called the Cambrian Explosion - the Big Bang of Biology.
"Geographically, about a billion years ago the continents were merged, forming one huge landmass called Rodinia, which broke apart 600 million years ago.
"You'll notice that this time period coincides nicely with the end of the ice-age previously mentioned, and immediately, again -- geologically speaking -- precedes both the advent of the Ediacaran Fauna and the later Cambrian Explosion.
"Okay, that's the general picture, in a nutshell, of what is known leading up to and surrounding the period in question. As you can see, we have a puzzle here; but rest assured, the final exam won't be for some time yet."
Nervous laughter reverberated through the room, releasing the tension. But it didn't last. The good Doctor had dumped a lot of information on us that most were probably unfamilar with. And in any event, what did this encyclopedic context have to do with the edifice? Mysteries of evolution, geologic and geographic transitional periods, atmospheric changes; no doubt -- sun spots, what else?
I could sense, being in the middle of it, that we were not sure at that moment if we'd learned anything. We were out of our collective depth.
Golgachev, looking both excited and tired, requested questions.
One voice asked for all: "Doctor, are you suggesting that the edifice, as it is being called, had something to do with all these coincident events?"
"Perhaps." he answered. "As I'm sure you agree, it's just too tantalizing to dismiss the possibility completely out of hand; although, obviously, we have no real reason to believe that it did, that is, no scientific reason, or even the vaguest idea, at this time, what influence the edifice, or rather, its occupants, if any, may or could have had on the events briefly outlined. Possibly, just a major coincidence among others of an earth-bound nature.
"Could be nothing, surely, but, it's still very early in our investigation. We have much to learn."