Could Humans Have Lived at the Time of Dinosaurs?

Gaian Regulation is not just a Question of Temperatures

The homeostasis of the Earth System, AKA “Gaia,” is often discussed in terms of planetary temperature regulation. But it is not just that. Whether human beings could have lived at the time of dinosaurs leads us to understand how the atmospheric composition was regulated to sustain large animals, “tetrapods,” for at least 350 million years. A manifestation of the capabilities of the Gaian System

The idea that our Stone Age ancestors could have been living during dinosaurs is a favorite sci-fi theme and the object of discussion among young Earth creationists. Apart from these realms, a little outside mainstream science, it is known that humans are separated from non-avian dinosaurs by some 65 million years; it is a little hard to think that they could have met!

Nevertheless, it is legitimate — and interesting — to ask whether humans could have actually lived at the time of dinosaurs. How about a time machine projecting you to the Jurassic? Would you need a respirator or a space suit?

Of course, the answer depends on what we know about the atmospheric composition in the remote past. First, pressure during the Phanerozoic Age (the past 540 million years or so) is believed to have remained constant, with nitrogen being the main component, as it is today. So, the main constraints would be the gases that humans breathe (oxygen) and emit (CO2).

Let me show you some data (Payne and Kaltenegger 2023). Note the large uncertainty in the data; it is due to the measurements being based on “proxies,” geologic traces left by processes that are affected by oxygen concentrations

Oxygen levels below 19.5% — not much lower than the current 21.5%, are already causing detectable hypoxia symptoms. But the human body is resilient, and you won’t faint until concentrations are below about 15%. If it goes below ca. 12%, you die. On the other side of the pressure range, oxygen can be toxic only when over ca. 50%.

So, our cleaning lady should have had enough oxygen during the dinosaur age (from ca. 170 my, to 65 My ago). She may have had a little too much in some periods, when she might have felt a little exhilarated. In others, she would have felt short of breath, but no more than that. There may have been exceptions, though. At the boundary between the Paleozoic and the Mesozoic, at the time of the “mother of all extinctions,” the oxygen level may have been too low for a human being to survive, but that’s not clear, given the uncertainty of the data. And, in any case, it was only for a short period in geological terms.

There is another side to the breathing question, and it is the carbon dioxide (CO2) pressure. O2 and CO2 go together. Plants need CO2, and for them, oxygen is waste. Conversely, animals need oxygen, while CO2 is waste. It is a delicate balance that was described as a “dance.” Oxygen transport in the body is regulated by CO2 concentration, and that’s the reason why, if you are stuck in a sunken submarine, you can’t survive by breathing faster. You will inhale a lot of oxygen, but it won’t be picked up by the haemoglobin molecules in your body, clogged by too much carbon dioxide.

The human body is tuned to live at a concentration of about 0.03% of CO2 (300 ppm). There are no known health effects of CO2 concentrations lower than that, but it is well known that high concentrations are bad for health. Human emissions have already brought the concentration to 420 ppm in the air. Inside buildings, you typically breathe 600-800 ppm. If your building has sealed windows (to “save energy”), you rapidly go to much higher ranges (also if you wear a face mask). More than 1000—2000 ppm (0.1%-0.2%) is considered unhealthy. More than 5000 (0.5%) is acceptable only for short periods; if you reach 5%, you die. Period.

So, regarding CO2 concentrations, our cleaning lady would have found the Mesozoic air somewhat stale and might have suffered from headaches, but she would have survived without a respirator. Only during the mid-Paleozoic or earlier would concentrations have been enough to cause damage, especially when coupled with low oxygen content.

In the end, yes: you could travel using time machines back up to some 350 million years ago, walk around, and maybe hunt dinosaurs for the sheer pleasure of it. And if we could manage to clone dinosaurs in our time, they could breathe our atmosphere, although they might find it low in oxygen, and they would tire easily.

Apart from science fiction, though, these considerations highlight how Earth’s ecosystem has seen a “tetrapod window” of some 350 million years when the atmospheric composition was compatible with large animals, except for the episodes of mass extinction. It is a remarkable example of “Gaian Regulation” that maintained these conditions good enough for more than 350 million years, and is still good. It highlights also the fundamental similarity of Earth’s organisms. They all use the same biological mechanisms. Inside their bodies, their mitochondria are busy at generating chemical energy using the same biochemical mechanism: the Krebs cycle. The same cycle powers mammals, dinosaurs, and cleaning ladies.

Note that these conditions never led to the development of today's highly encephalized creatures. Humans could survive during the Cretaceous, but they wouldn’t have been able to evolve at that time. Dale Russell's “dinosauroid” was an interesting exercise of how an intelligent dinosaur could have evolved, but it remained theoretical. There was no way at that time to oxygenate a brain with characteristics similar to the human one.

Only during the past 30 million years or so we saw a change, with CO2 going down to levels never seen before. It may be because of this low concentration that it was possible to evolve large brains with a high neuron density, such as the human one. Now, of course, we are reversing everything with CO2 emissions, dumbing ourselves back to the age of dinosaurs. Except that we aren’t dinosaurs. But that’s another story that I told in a previous post.

_________________________________________________________

On this subject, take a look at this excerpt from the recent paper “Evolution of Atmospheric O₂ Through the Phanerozoic, Revisited” and see if you can spot the mistake they are making.

When considering the evolution of intelligence, a useful question is, “What is the earliest point in Earth history during which humans could have survived on Earth's surface?” The answer would give an indication of what kind of biosphere was needed on our planet to support intelligence as we know it, and therefore what evolutionary advances might be needed for intelligence to arise on an exoplanet. Currently, the highest altitude of long-term human colonization is on the Tibetan Plateau at around 4,500 m, where the density of the atmosphere is around 60% of that at sea level. The partial pressure of oxygen experienced here is equivalent to being at sea level on a planet with only ∼12.5% O₂ in its atmosphere. A selection of metabolic traits acquired over more than 6,000 years of permanent human habitation of the plateau (Horscroft et al. 2017) have made complex, intelligent life here possible. This level of atmospheric oxygen abundance is roughly what is thought to be required for oxygen to penetrate deep into the ocean interior (Canfield 1998). Thus, given that our main constraint on pre-Silurian atmospheric oxygen levels is the persistence of marine anoxia (Sperling et al. 2015), it is possible that an atmospheric oxygen concentration sufficient for highly adapted modern humans may have been available throughout the entire Phanerozoic and perhaps even into the Proterozoic.

I am not disparaging the paper or the authors. But this paragraph is all wrong: people could NOT have lived during the pre-Silurian; no way. At high altitudes, the oxygen pressure is reduced, but its fraction in the air remains the same 21% as it is at sea level. So, you can compensate by breathing faster and developing larger lungs. During the Silurian, and most of the Phanerozoic, instead, reduced oxygen levels were associated with high CO2 levels. And people can’t survive for an oxygen/CO2 ratio lower than a minimum level. Just to note how easy it is for all of us to miss some fundamental points when moving away from one’s field.