If the gravity on earth were different, how would the human body change?

Question

If the gravity on earth was different, how would our bodies change?

For this question, assume that gravity was five time stronger (5G) from the beginning of the Earth. Would a human-like body still be able to evolve? If so, how would it differ from the bodies we have?

Answer 1

If gravity were five times as strong as it is now, I'd be more concerned with whether or not our universe still existed.

Let's assume you mean that the mass of the earth was five times greater than its present mass, so that its gravitational pull would be correspondingly higher. The effects would be environment wide, so rather that focus on humans, lets look at the general situation. Here's my guess :

• Flying Animals These would be extremely unlikely. The extra energy required to maintain flight would demand extra strength ( = extra mass ), and conversely, the extra mass would require extra energy. The costs may exceed the benefits, so evolution might not take hold in flying animals. Some gliding by land based animals may evolve.
• Land Based Aminals Smaller animals would be favoured by evolution since they would require less energy to move efficiently in a strongly gravitational environment. The skeletal mass of animals would need to be proportionately higher in order to maintain whatever reduced height was optimal. Similarly, muscle mass would need to be proportionately greater if high mobility was required. Most animals would be slow moving, with perhaps a few species of predators having speed. I would guess that a low, flat form with many supporting legs would be the most common body design, similar to bugs - think centipede.
• Water Based Animals Again, stronger gravity would favour bottom dwelling aquatic species. The amount of energy needed to swim freely would exceed the amount of energy available from food floating freely (plankton), and free floating food would sink faster. Again, one assumes that a flat form with larger skeletal mass would be the most common form. Localized underwater currents may provide some extra variety. Salt water is more buoyant that fresh water, so the changes would be more pronounced amongst fresh water species. The surface of the oceans may support life, perhaps in the form of a paper-thin animal with a large surface area and featuring thing tentacles descending into the waters to feed on plankton forming in the sunlit surface waters.
• Plant Life While height may be common, side branching would be very difficult and expensive to maintain in terms of energy consumption. Plant life would be unlikely to bend in the wind since they would require extra rigidity to support their weight. Once again, low growing, flat, well supported forms would be dominant.
• Geology and Climate The extra gravitational pull of the earth would make it more difficult for high mountains to form. This would mean the surface would tend to be flatter which may imply less dry land and more surface area covered by ocean. Rivers would be less mighty. The climate would be hotter. This is because the earth itself would be denser so its core would be hotter. Also contributing to a hotter planet would be the extra energy required to lift water vapour from the seas into the atmosphere, perhaps resulting in fewer clouds and less rain. This extra energy could come from a more turbulent atmosphere, but that seems unlikely since a smoother planetary surface (no big mountains) would actually reduce atmospheric turbulence.

So ya, things would be shorter and fatter. Kind of like living in Alabama.

EDIT Reading sixfootersdude's comments below, I think I have overlooked an atomospheric consequence of this scenario. The atmosphere would hug the earth much closer than it does currently. This added density would result in added heat since molecules would collide much more frequently. It this is the case, then one would expect to see many more cloud and much more rain, rather than the drier climate I had originally assumed. One suspects that visibility would be severely restricted.

I don't see how a denser atmosphere would be more helpful for flight. Gram for gram, animals would weigh five times as much. The atmosphere would need to be many thousands of times denser (possibly more) to allow for the type of flight described. This would make the atmosphere much more difficult to move through, further hindering flight and all types of motion by animals.

Answer 2

Bigger and slower (though to that creature, it'd be normal and we'd be smaller and faster).

5G doesn't simply effect the weight of creatures...it effects every element on the globe. Air pressure become significantly stronger and heavily impacts the resistance of the atmosphere to movement. Rain becomes heavier, but has to battle the higher pressure of the atmosphere to fall (terminal velocity changes). Oxygen at 5g (assuming this results in 5x the atmospheric pressure) actually becomes toxic to life as we know it at, just from the pressure of the atmosphere. Water boils at 120 degrees from simply doubling the pressure on it, however the increased pressure would also force down the melting point of water significantly.most of the equilibrium's that balance our body change under pressure as well. Life would have to be fundamentally different at a chemical level...pressure heavily effects photosynthesis, so chlorophyll plants may never come to dominate. Blood pressure would have to be significantly higher to ensure circulation...Oxygen content in your blood (both from regular content vs oxygen saturated) is heavily based on pressure. In short, pressure effects the equilibrium of nearly every chemical process known life relies on.

My guess is creatures would be larger and slower. Not sure if I can speculate beyond that. This is also my answer to speculation on what other chemistrys life could be based on...in different gravitys when chemical properties change, we really have no clue what chemical processes can drive life.

Answer 3

While it is difficult (read: impossible) to predict what this would mean for a world, I'm going to go ahead and guess a 'no' on humans existing. Human ancestors were primarily tree-dwelling mammals and in a world with 5G, trees become pretty darn dangerous.

Imagine falling out of a tree. That hurts right, you might even kill yourself. Now imagine falling out of a tree weighing 400 kg. You're going to hurt yourself, a lot.

Massive dinosaurs would also be unfeasible in high gravity environments, and their lack may very well mean mammals end up dominant way earlier. I would assume that in a high gravity environment, almost all life will be low-to-the-ground to avoid falling related injuries.

Answer 4

Here is a late answer: most life would be in water as water based life would have no problem with gravity. Buoyancy is not affected by gravity. Additionally, internal and external pressures would be balanced. Circulation systems would need more pressure to work against higher pressures. But then again, water cannot be compressed much. Resulting in a manageable change.

All in all, the ones that got hit would be land animals. Being on land could be so costly that in your world there might be no land animals at all. Or just small bug type animals with short plants would cover the face of the earth.