Archaeologists do not know for sure which species actually created and used Oldowan tools. The use of these tools reached its peak with early species of "Homo," such as "H. habilis" and "H. ergaster". Archaeologists think that early "Homo erectus" inherited Oldowan technology, then refined it into the Acheulean industry beginning about 1.7 million years ago. Oldowan tools are sometimes called "pebble tools", because the blanks chosen for their production already resemble, in pebble form, the final product. They are sometimes subdivided into types, such as choppers, scrapers and pounders, based on their main uses.