I will give you some straightforward facts and observations. Then I will draw a conclusion that is utterly unreasonable and probably untrue. The hugely popular book The World Without Us (Weisman 2007) discusses what might happen to the earth if humans disappeared tomorrow. Weisman argues that after only a few centuries, few if any traces of us would be left. A similar book, The Earth After Us (Zalasiewicz 2009) speculates about traces of mankind in the future geological record. A hundred million years from now there would be little direct evidence that we were here. Our 100 000 years of existence as a species would be represented by perhaps a metre of stratigraphy. Through most of that time, we were exceedingly rare and did not fossilize well because of our terrestrial habitat. Cities and dams may last for a while, but eventually they will erode or be tectonically subducted.
As geologists we have been taught that ‘the present is the key to the past’. Uniformitarianism, although not universally applicable, has been an invaluable tool for understanding geological processes in deep time. It could be suggested that we are now in a period of mass extinction caused by the actions of an intelligent species. We know of several mass extinctions in the geological past — by the principle of uniformitarianism, what should be our null hypothesis for the cause of these previous extinctions?
Zalasiewicz suggests that a negative carbon isotope excursion will be one of the few long-term traces of the human episode. Some previous mass extinctions, in particular the event at the end of the Permian, but also some minor events such as the end of the Palaeocene, are associated with precisely such negative carbon isotope excursions. Again, by the principle of uniformitarianism, what should be our null hypothesis for the cause of these excursions?
By now, it may have dawned on you what I’m getting at (of course, just for the record, being a serious man and fond of my job, I don’t really mean it). I’m saying that it would not be surprising if civilizations in the geological past would have been hitherto unnoticed by geologists and palaeontologists. Instead of searching for alien civilizations in outer space, we might have better chances searching for them on Earth back in time, where we demonstrably had ecosystems to support them. Where should we look? In marine sediments perhaps, where remains of their magnificent sunken ships might have been preserved. In ancient hydrocarbon reservoirs perhaps, where their mighty drilling equipment could have fossilized. Or even better perhaps (as suggested to me by physicist Galen Gisler), on the moon and Mars, where their scientific landers might have remained unscathed through millions of years.
The idea that evolution is progressive is now out of fashion among biologists. Humans are no longer considered to be at the top of the heap. Evolution has not been a long march with the sole purpose of producing us as a finale. So why should we believe that nothing similar has happened before? To claim that we are the only intelligent species through time has the same philosophical status as claiming that we are the only intelligent species in the universe. It’s unscientific.
Hang on, you say. Maybe we would not find their buildings or skeletons, but surely we should find their evolutionary precursors? Not so. Consider the fossil record of primates. It is almost non-existent, just some scraps here and there. Another 50 million years of erosion and subduction, and it will be virtually gone. Here is one possible scenario, out of many: there was a tree-living synapsid in the Permian, called Suminia. Here it is, below, together with the primate Ida (Darwinius masillae) from the Eocene. Which is which? Give Ida 50 million years, and she turns into us. Give Suminia 50 million years, and we are in the late Triassic. Could she have turned into something similar? Yes. Would we have fossil evidence for such an evolutionary sequence? Probably not.
Think about it. But don’t quote me on this.
Franzen, J L, P Gingerich, J Habersetzer, J Hurum, W von Koenigswald, H Smith (2009). Complete primate skeleton from the Middle Miocene of Messel in Germany: Morphology and paleobiology. PLoS ONE 4 (5) e5723.
Fröbisch, J and R Reisz (2009). The Permian herbivore Suminia and the evolution of arboreality in terrestrial vertebrate ecosystems. Proceedings of the Royal Society B, 276, 3611–3618.
Weisman, A (2007). The World Without Us. Thomas Dunne Books / St Martin's Press, 336 p.
Zalasiewicz, J (2009). The Earth After Us. Oxford University Press, 272 p.