The 1-Up Heart was a replacement for the 1-Up Mushrooms in the first two Super Mario Land games for the original Game Boy. The only difference between the 1-Up Heart and the 1-Up Mushroom is that the 1-Up Mushroom scrolls across the landscape after being found, while the 1-Up Heart stays still. Unfortunately for the 1-Up Heart, after Super Mario Land 2: 6 Golden Coins, the 1-Up Heart was never to be seen again, as the 1-Up Mushroom went on to be the great iconic life producing Mario item.
It has been assumed that the inclusion of 1-Up Hearts can be attributed to the limited color producing abilities of the Game Boy. Since Super Mushrooms were already present in the game, it would've been hard to distinguish two similar looking mushrooms apart, thus giving birth to the 1-Up Heart. It is also thought that this is why the item never went on to be featured in later Mario series titles, as the 1-Up Mushroom could be distinguishable on every later console. It is also interesting to note that 1-Up Hearts are also seen in Balloon Kid, also on the Game Boy.