Reddit mentions of Masters of the Planet: The Search for Our Human Origins (MacSci)

To make things as clear as possible, there is no absolute way for us to know with our current evidence how these extinct hominins were related to one another. We can surmise based on where and when the fossils are found, and possible links between morphologies of different specimens but in all honesty, these are just hypotheses. Two major reasons why it's so hard to accurately place these fossils:

1. Only a fraction of the living species present at the time are fossilized. It's on the order of around 5%. So take a walk in your woods today, at best only 5% of those species will fossilize.
   
   
2. Those species which are lucky enough to get fossilized have to eventually be exposed in a location where humans are going to find them.
   
   All this means is that it is very unlikely that we have found all the hominin species that ever existed.
   
   There are two major camps which exist in regards to how these fossils are categorized. First there are the lumpers and second there are the splitters.
   
   
3. Lumpers: this group likes to classify all the fossils from around 2.5 mya years ago until today as a single species which has changed slowly over time. So rather then having H. erectus you would have H. sapiens erectus. They argue that the morphological differences and corresponding behavioural attributes (e.g. tool cultures) are not different enough to warrant separate species classification.
   
   
4. Splitters: They like to divide the fossils up based on morphological, behavioural, spatial and temporal divisions. The number of species varies because not everyone agrees on the amount of difference required for species status. Needless to say a few hypotheses stand out and I will go over them here. Wikipedia is a great resource for starting off, but learning the terminology can be tricky.
   
   Hypotheses regarding the origins of H. erectus
   
   
5. H. erectus is a descendant of H. ergaster. H. erectus fossils are all found in Eurasia, whereas H. ergaster fossils are all found in Africa. H. ergaster evolved in Africa about 1.8 million years ago and quickly gave rise to H. erectus in Eurasia. H. erectus then spread as far as China and Java exploiting many new environments within this vast territory.
   
   
6. H. ergaster and H. erectus are the same species and share a common ancestor in Africa. The only distinction is that H. ergaster would refer to African fossils of this species and H. erectus would refer to the Eurasian ones. This seems to be the more favourable hypothesis right now.
   
   
7. It is unlikely that H. habilis is the direct ancestor of H. ergaster or H. erectus or both. First, it looks like H. habilis coexisted with ergaster/erectus in Africa for a couple hundred thousand years. More recently H. habilis has come under closer scrutiny and there are those would would have it placed not as the first known member of the Homo species, but rather as an australopithecine.
   
   "In 2007, new findings seemed to confirm the view that H. habilis and H. erectus coexisted and may be separate lineages from a common ancestor instead of H. erectus being descended from H. habilis.[6] At the very least, these findings indicate that any ancestral relationship from H. habilis to H. erectus would have to have been cladogenetic rather than anagenetic (in more vernacular terms, this means that even if an isolated subgroup population of H. habilis did indeed become the common ancestor of the rest of the genus, other subgroups remained as unchanged H. habilis until their much later extinction".
   
   External references
   
   
8. How humans evolved
   
   
9. Masters of the planet
   
   
10. Smithsonian human evolution, more specifically on H. habilis: "While scientists used to think that H. habilis was the ancestor of Homo erectus, recent discoveries in 2000 of a relatively late 1.44 million-year-old Homo habilis (KNM-ER 42703) and a relatively early 1.55 million-year-old H. erectus (KNM-ER 42700) from the same area of northern Kenya (Ileret, Lake Turkana) challenged the conventional view that these species evolved one after the other. Instead, this evidence - along with other fossils - demonstrate that they co-existed in Eastern Africa for almost half a million years."
    
    Tl;DR Current evidence suggest that H. erectus is not the direct descendant of H. habilis

Masters of the Plant is a pretty decent one. The author, Ian Tattersall, is a Cambridge grad with a PhD from Yale, and is curator emeritus with the American Museum of Natural History.

Some great answers in here, but if you really want to understand then you will want to dive into a documentary, lecture, or book.

Documentary: https://www.youtube.com/watch?v=C7WHs6I1NLs

Lecture: https://www.youtube.com/watch?v=x858bOny4Gw

Audiobook: https://www.youtube.com/watch?v=ZG0j_lvW6A0

Books!

Relatively light reading: https://www.amazon.com/Masters-Planet-Search-Origins-MacSci/dp/1137278307/

A bit higher level: https://www.amazon.com/Lone-Survivors-Came-Humans-Earth/dp/1250023300

Textbook: https://www.amazon.com/Complete-World-Human-Evolution-Second/dp/0500288984