With Coinbase you can use USD to buy LTC, BTC, and ETH directly yes. Though if we look around the total crypto space we are seeing more and more global exchanges that are opening up direct buying opportunities for fiat pairs. so one could skip bitcoin and go straight into Dash, or Z-Cash, or Ethereum classic if they choose. This is happening increasingly on the Chinese and Korean exchanges. Bitcoin is still the whale that's true, but it's dominance is being chipped away at in a steady way and will continue to do so until it finally scales and adopts segwit. If that whole thing gets solved in a satisfying way, which I'm sure it will, bitcoin will see it's nice stairstep. We will probably get an other surge into alt's at the same time.

@dandesign86 If you are going to invest in bitcoin right now just be SURE to read up about Segwit2x (google it or search here on steemit) and what the implications are and HOW and WHERE you should store your bitcoin (hint, not on an exchange like coinbase or poloniex. Electrum wallet seems to be a very good safe storage place)