Good job on your first game!

• I had a couple of issues, the fire rate was pretty slow and especially on level 2 it felt pretty impossible to shoot all of the enemies, it'd be nice to see how long it takes for you to reload.
• The difficulty curve could also be a little better, most levels were pretty easy and the same but level 2 was really hard.
• On some levels the path of the enemy was a little unclear, so it would be nice to show a path like you had in the other levels to see how long it'll take for the enemies to reach the end.
• I think the pause menu in the beginning should probably be a start menu, maybe you could explain some stuff in the beginning as well like what the evolution speed is.
• In level 4 the gun shot over my crosshair, and the timer ran out with no ending.
• The third level only had one enemy for me.
• The theme was a little unclear, you do change guns between each level but they seemed to be pretty much the same, the variation in sounds and sprites is nice though.

Aside from that, I think you did a pretty good job on your first game, especially in such a short time frame!

Congratulations on your first game! Here is some feedback: 

• the gameplay feels very slow and gets monotonous very fast. Maybe you could speed it up a notch and try to change it up a little from time to time, for example by including temporary upgrades, special enemies, events or some other abilities, that the player can use.
• it would be nice to have some indicator, how long it will take until the weapon is reloaded.
• I think it is clear that there is a lot of room for improvement regarding the visuals. At the moment, it's hard to even make out what everything is supposed to be. For example, apart from reading the description, I would not have known that I am a time traveler. I am generally a fan of games that show, don't tell, i.e. don't need a wall of text to convey information. But for your first game (and in 3 hour game jams) this is sufficient. Also, the gradient on top of the progress bars made it hard to read the text (evolution/time remaining)
• the second level was already really hard. As nardandas already pointed out, it was really easy to miss the enemies and it would take too long to reload.
• A main menu is usually better than throwing the player into a pause screen. But I guess it works too.
• If I am not mistaken, there was no way to upgrade the weapon, so it did not really fit this jam's theme. While the theme is optional and not mandatory, your game will receive more attention during the jam if it fits the theme.
• As a general hint, although you already provided a web build: for game jams, it is really beneficial to have a web build of your game. You will lose a lot of players (and therefore ratings) already, if you have to download something first

Good luck for your second game!

Cool idea, though I found the fire rate far too slow to be usable. Particularly on the second level I felt I couldn't shoot fast enough to kill the things even if I landed each shot.