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On my world, the equivalent of the Cambrian/Avalon explosion happened 5 byr (2 byr after formation) ago as opposed to 0.5 byr ago on Earth.

The planets rough parameters are:

  • mass between 0.4 and 0.25 Earth-masses

  • only 10 to 30 percent global ocean cover, water is mostly near the poles

  • orbits a 10.9 Jupiter-mass warm super-Jovian

  • extensive vulcanism and some plate tectonics

  • the global average temperature is 63 C, just shy of a runaway greenhouse effect given the local atmospheric pressure of 2,7 atm

  • only one atmospheric circulation cell due to slow rotation (the moon is tidally locked)

  • long days and nights (still need to do the math, assume one terra-week each)

  • the sun is an old F-type with only a few hundred million years left on the main sequence

So how would this planet's 5 byr old biosphere and environment be different from Earths young one in a broad sense?

I´m aware that this could easily be considered as too broad or opinion based, but I´m only looking for macro details. For example, I think that the planet would have very extensive fossil fuel deposits, a very diverse tree of life, an environment where life has adapted to very exotic niches (cloud-algae for example),... .

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    $\begingroup$ There is no way to tell; it is pure unadulterated speculation. Nobody can give any "macro details" (is a macro detail sort of like a micro trend?) with any degree of certainty. You may want to ask about speculative evolution on the appropriate site; there are quite a few books on the subject, starting with Olaf Stapledon's Last and First Men (1930), up to C. M. Kosemen's All Tomorrows (2008). $\endgroup$
    – AlexP
    Jul 20, 2019 at 14:48
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    $\begingroup$ It will be equal but different. Evolution is all about context, If you put dinosaurs and mammalian megafauna together who dominates is determined by the environment not them. Latter evolved groups do not have an advantage over younger ones. That is the only macro-detail you can get. How long it has been since the last mass extinction will matter far more than the age of the planet. $\endgroup$
    – John
    Jul 20, 2019 at 15:08
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    $\begingroup$ You're assuming a planet that has a history of multicellular life longer than the entire geological history of our own planet, too, so imo the biggest thing you need to account for is not the current condition of the planet but the ways in which it has changed over that time. These changes should be quite radical, varied, and have diverse causes (i.e. large meteor impacts, nearby supernovae, perhaps extreme changes in the local star, and probably many more atmospheric changes than Earth has experienced in its history.) $\endgroup$
    – SudoSedWinifred
    Jul 20, 2019 at 15:14
  • $\begingroup$ You say "On my world, the equivalent of the Cambian/Avalon explosion happened 5 byr (2 byr after formation) ago as opposed to 0.5 byr ago on Earth." Is that a typo? 5 bya vs 5 bya? Or does the Earth time refer to what's in parentheses? $\endgroup$
    – Cyn
    Jul 20, 2019 at 15:14
  • $\begingroup$ The Ediacaran/Cambrian explosion occurred when the Earth was around 4 billion years old. So are you saying that yours occurred in half as long? $\endgroup$
    – Arkenstein XII
    Jul 20, 2019 at 23:26

2 Answers 2

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I suspect things would play out in one of two ways:

1. Familiar But Different

Just as Earth's biosphere has gone through cycles of boom and bust, your world would face periodic mass extinctions from widespread volcanic eruptions, impact events, Great Oxidization Event-equivalents, and so forth. Assuming similar numbers of species survive each time, and these extinctions are spaced out comparable to our own, you'd have a planetary biosphere no more advanced or primitive than any given point in Earth's own history.

2. Harder Better Faster Stronger

This is a world without mass extinctions for quite some time.

Everything we know of evolutionary adaptation and trends would be taken to higher orders: parasites within parasites within parasites, venomous attacks and defences, convergence in niche specialization and generalization, adaptive radiation, symbiosis of greater and greater complexity with increasing numbers of species, smaller and smaller niches, and so on.

Note

A terrestrial world with only 10–30% surface water is going to be mostly deserts. If oceans are concentrated mainly at the poles you're likely to have a largely unbroken band of desert (sandy, rocky, hot, or cold) from 45°N to 45°S (or more). Recall that deserts are defined by preciptiation, not heat.

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    $\begingroup$ In a world without mass extinctions, you get extreme specialization. Extreme specialization also means extreme fragility: it's the generalists who survive changing conditions, not the specialists. $\endgroup$
    – Mark
    Jul 23, 2019 at 1:49
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The planet's life-forms have gone through several stages:

  • Natural development, natural environmental factors such as repeated ice-ages and extinction level events have produced over a significant timeline - a diverse range of species. Particularly in isolated island environments - much like Birds-of-Paradise, or Chichlid populations have developed on Earth.

  • Intelligence emerges dominant over it's natural predators. The predominant life-form has developed intelligence and culture to the point of sapience, agriculture and animal husbandry. Significant areas of land are used for crops, increasingly tending towards mono-cultures, as immunity to various pests is sought. Animals are bread, the natural stock's diversity of genome is subsumed by the need to develop commercial "big and muscular" stock that breed fast without hassle.

  • Industrial revolution. Vast swaths of natural resources are used to power the transformation to an industrial society, commerce exerts it's pressures. The atmosphere begins to change.

  • Pollution, extinctions, sudden climate change, drought and famine become global issues as the climate sways towards the unstable.

  • The dominant species gets a grip and re-balances the bio-sphere in a way favorable to the survival of (predominantly themselves) life on your planet. Somewhat through nano-tech, some through the transformation of energy usage, some through bio-engineering. (This is a phase of great danger to your dominant species) Many extinctions, and the chance of utter failure must be dealt with.

  • The entire bio-sphere is engineered to take advantage of all the incident energy from your star and the geo-thermal energy. This is done (quite automatically by means of biologically engineered micro-organisms) to take advantage of every niche that an organism might fill (see convergence of biology and engineering). Your entire planet is engineered to be optimal for life, and life is engineered to be optimal for the planet - perfect synergy.

This is only one possibility of how the planet's development might occur, and any development after this point would delve even further into speculation. The alternatives would seem to either be that intelligence doesn't develop, it does and destroys itself or gets wiped-out before it has sufficient power to prevent it. There seems sufficient time in your scale for it to die-out and re-develop several times. It would seem inevitable that if it survives, it should have extended off-planet maybe even a couple of billion years before the end of the time-scale that you suggest.

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  • $\begingroup$ The idea of civilisations rising and falling is neat, yet in the context if my setting it us not possible. I decided that the solution to the Fermi-paradoxon is very inevitable abiogenesis, rare eukaryotes (mean time for them to occur in a planet is 10 byr) and extremely rare intelligence capable of devising complex tools (within mny byr of human expansion only 7 intelligent spacefaring species have been found). So having civilisations arises two times on the same rock is impossible. $\endgroup$
    – TheDyingOfLight
    Jul 21, 2019 at 8:12
  • $\begingroup$ @TheDyingOfLight fair enough. I suddenly realized that I'd left out this link to early Earth's atmosphere with refs to methane and ammonia, but you were probably aware of it anyhow. $\endgroup$
    – Angry Muppet
    Jul 21, 2019 at 8:23

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