Why are humans here? No, I’m not going to talk about any gods being responsible for our placement on this planet, though some extraterrestrial phenomena do play a part in why we are here today. The story of how and why we are here is extremely fascinating, because we are here only by chance, not by any divine purpose.
To understand why we are here, we first need to know what we evolved from and where this organism evolved. The Burgess Shale is a limestone quarry formed after the events of the Cambrian explosion. In the Shale are the remnants of an ancient sea that had more varieties of life than today’s modern oceans. The Shale is the best record we have of Cambrian fossils after the Cambrian explosion we currently have. Preserved in the Shale are a wide variety of creatures. One of these creatures is our ancestor, the first chordate. It’s name: Pikaia gracilens.
Pikaia is the only fossil from the Burgess Shale we have found that is a direct ancestor of humans. Now think about the Burgess decimation and the odds of Pikaia surviving. If this one little one and a half inch organism didn’t survive the Burgess decimation, everything you see around you today would not be here. By chance, we humans are here today due to the very unlikely survival of Pikaia. Stephen Jay Gould wrote a whole book on the Burgess Shale and ended his book Wonderful Life: The Burgess Shale and the Nature of History (1989: 323) as follows:
And so, if you wish to ask the question of the ages—why do humans exist?—a major part of that answer, touching those aspects of the issue that science can touch at all, must be: because Pikaia survived the Burgess decimation. This response does not cite a single law of nature; it embodies no statement about predictable evolutionary pathways, no calculation of probabilities based on general rules of anatomy or ecology. The survival of Pikaia was a contingency of “just history.” I do not think that any “higher” answer can be given, and I cannot imagine that any resolution could be more fascinating.
The survival of organisms during a mass extinction may be strongly predicated by chance (Mayr, 1964: 121). The Burgess decimation is but one of five mass extinction events in earth’s history. Let’s say we could wind back life’s tape to the very beginning and let it play out again, at the end of the tape would we see something familiar or completely ‘alien’? I’m betting on it being something ‘alien’, since we know that the survival of certain organisms is paramount to why Man is here today. Indeed, biochemist Nick Lane and author of the book The Vital Question: Evolution and the Origins of Complex Life (2015) agrees and writes on page 21:
Given gravity, animals that fly are more likely to be lightweight, and possess something akin to wings. In a more general sense, it may be necessary for life to be cellular, composed of small units that keep their insides different from the outside world. If such constraints are dominant, life elsewhere may closely resemble life on earth. Conversely, perhaps contingency rules – the make-up of life depends on the random survivors of global accidents such as the asteroid impact that wiped out the dinosaurs. Wind back the clock to Cambrian times, half a billion years ago, when mammals first exploded into the fossil record, and let it play forwards again. Would that parallel be similar to our own? Perhaps the hills would be crawling with giant terrestrial octopuses.
I believe contingency does rule—we are the survivors of global accidents. Even survival during asteroid impact and its ensuing effects that killed the dinosaurs 65 million years ago was based on chance. The chance that the mammalian critters were small enough and could find enough sustenance to sustain themselves and survive while the big-bodied dinosaurs died out.
Let’s say one day someone discovers how to make a perfect representation in a lab that perfectly mimicked the conditions of the early earth down to the tee. Let’s also say that 1 month is equal to 1 billion years. In close to 5 months, the experiment will be finished. Will what we see in this experiment mirror what we see today, or will it be something completely different—completely alien? Stephen Jay Gould writes on page 323 of Wonderful Life:
Wind the tape of life back again to Burgess times, and let it play again. If Pikaia does not survive in the replay, we are wiped out of future history—all of us, from shark to robin to orangutan. And I don’t think that any handicapper, given Burgess evidence known today, would have granted very favorable odds for Pikaia.
Why should life play out the exact same way if we had the ability to wind back the tape of life?
Another aspect of our evolution and why we are here is the tiktaalik, the best representative for a “transtional species between fish and land-dwelling tetrapods“. Tiktaalik had the unique ability to prop itself up out of the water to scout for food and predators. Tiktaalik had the beginnings of beginnings of arms, what it used to prop itself up out of the water. Due to the way its fins were structured, it had the ability to walk on the seabed, and eventually land. This one ancestor of ours began to gain the ability to breathe air and transition to living on land. If all tiktaaliks had died out in a mass extinction, we, again, would not be here. The exclusion of certain organisms from history then excludes us from the future.
And now, of course, with talks of the how and why we are here, I must discuss the notion of ‘evolutionary progress‘. Surely, to say that there is any type of ‘progress’ to evolution based on the knowledge of certain organisms’ chance at survival seems very ludicrous. The commonly held notion of the ‘ladder of progress’, the scala naturae, is still prominent both in evolutionary biology and modern-day life. There is an implicit assumption that there must be some linear line from single-celled organisms to Man, and that we are the eventual culmination of the evolutionary process. However, if Pikaia had not survived the Burgess decimation, a lot of the animals you see around you today—including us—would not be here.
If dinosaurs had not died out, we would not be here today. That chance survival of small shrew-like mammals during the extinction event 65 mya is another reason why we are here. Stephen Jay Gould (1989) writes on page 318:
If mammals had arisen late and helped to drive dinosaurs to their doom, then we could legitamately propose a scenario of expected progress. But dinosaurs remained domininant and probably became extinct only as a quirky result of the most unpredictable of all events—a mass dying triggered by extraterrestrial impact. If dinosaurs had not died in this event, they would probably still dominate the large-bodied vertebrates, as they had for so long with such conspicuous success, and mammals would still be small creatures in the interstices of their world. This situation prevailed for one hundred million years, why not sixty million more? Since dinosaurs were not moving towards markedly larger brains, and since such a prospect may lay outside the capability of reptilian design (Jerison, 1973; Hopson, 1977), we must assume that consciousness would not have evolved on our planet if a cosmic catastrophe had not claimed the dinosaurs as victims. In an entirely literal sense, we owe our existence, as large reasoning mammals, to our lucky stars.
He also writes on page 320:
Run the tape again, and let the tiny twig of Homo sapiens expire in Africa. Other hominids may have stood on the threshhold of what we know as human possibilities, but many sensible scenarios would never generate our level of mentality. Run the tape again, and this time Neanderthal perishes in Europe, and Homo erectus in Asia (as they did in our world). The sole surviving stock, Homo erectus in Africa, stumbles along for a while, even prospers, but does not speciate and therefore remains stable. A mutated virus then wipes Homo erectus out, or a change in climate reconverts Africa into an inhospitable forest. One little twig on the mammalian branch, a lineage with interesting possibilities that were never realized, joins the vast majority of species in extinction. So what? Most possibilities are never realized, and who will know the difference?
Arguments of this form led me to the conclusion that biology’s most profound insight to human nature, status and potential lies in the simple phrase, the embodiment of contingency: Homo sapiens is an entity, not an idea.
In any type of rewind scenario, any little nudge, any little difference in the rewind would change the fate of the planet. Thusly, contingency rules.
So the answer to the question of why humans are here doesn’t have any mystical or religious answer. It’s as simple as “No Pikaia, no us.” Why we are here is highly predicated on chance and if any of our ancestors had died in the past, Homo sapiens would not be here today. Knowing what we know about the Burgess Shale shows how the concept of ‘progress’ in biology is ridiculous. Rewinding the tape of life will not lead to our existence again, and some other organism will rule the earth but it would not be us. The answer to why we are here is “just history”. I don’t think any other answer to the question is as interesting as cosmic and terrestrial accidents. That just makes our accomplishments as a species even more special.