system
Given the following post, respond as if you're commenting on Reddit:
human
[Mass Effect] Are all the races descendants of the Leviathans?

According to what I can gather from the back-story, the Leviathans created the Reapers in order to maintain a balance between synthetic and organic life, but I'm not sure if this implies that xenocide was intended. Perhaps the Leviathans alone expanded across the galaxy and then when the Reapers systematically wiped them out to (near) extinction, the separate Leviathan factions evolved differently based off the environment of their home planet.

This would explain the humanoid similarities between the races, and simplify the existence of nearby mass relays without the Reapers having to do major reconnaissance work.
gpt
No, the Leviathans were the race that created the Catalyst AI to resolve the problem they had of constant wars between organic Leviathans and synthetics (robots/AIs).

The Catalyst created the Reapers, a fusion of organics and synthetics.  The Leviathans were the first harvested by the Reapers.  Almost all Leviathans but a small group, hidden away, were wiped out. 

50,000 years later, new species had arisen and also fallen into conflict with their synthetics.  So the Reapers harvested them too.  The Citadel and Mass Relays were built to make harvesting of future races easier - if you give species an 'easy way' to expand, most of their population will expand along those lines.  Thus their location and tech level are predictable.

The Leviathans stayed hidden away - there aren't enough of them to carry on a population.  They are relics - each generation suffering more and more inbreeding damage, if they breed at all.

In the Prothean Cycle, everything went as it had in all previous cycles. One race arose to dominate all the others.  In their final throw of the dice, the Protheans meddled with the Asari, intending to turn them into the conquerers of the next cycle.  They also closed the Citadel Relay, intending to make it harder for the Reapers to begin the next harvest.

But something went wrong.  The Asari didn't become conquerers.  They became mediators.  The Asari cycle didn't have one galaxy-spanning empire, they had an alliance of dozens of different species - each with their own leadership, own forces, own technologies.

The Reapers's job became much, much harder.  But even then, too much was centralised on the Citadel.  Too much technology was based on the 'easy way' of the Mass Relays.  And ultimately, too much time was wasted after Sovereign's attempt to open the Citadel Relay.

The Asari cycle was also doomed.  Until a last-ditch effort by the Alliance and a Special Forces soldier called Commander Shepard managed to attach The Crucible to the Citadel.  The Crucible was essentially a huge power cell and processing databank that allowed the Catalyst to calculate a different solution than the Reapers.