Hi George and thanks for your interesting question about early humans. I'm not sure I follow everything, but I'll do my best.

Indeed, humans have a long fossil record in Africa. Unfortunately, it can be very difficult to determine the hunting prowess of extinct animals. Even for an animal as well known as Tyrannosaurus, whether it was a scavenger or active predator is hotly debated.

Many early species of human are extremely poorly known--the number of specimens available to be examined are limited. Even if fossils were more common, however, the skeleton of primates offers relatively few clues about "hunting" abilities.

In any case, I don't know that the disappearance of any large carnivores has been tied to early humans. Saber tooth cats are a slightly later, and thus different story. Smilodon, the saber-tooth cat, lived in the Americas until about thirteen thousand (not million) years ago, which is practically yesterday in geological terms. Humans reached the Americas at almost exactly this time. This has led many scientists to argue that extinction of various large mammals (megafauna) at that time (mammoths, horses, ground sloths, etc.), was caused by overhunting by these first Americans. The elimination of large herbivores, would have naturally led to the disappearance of the species that ate them, such as Smilodon.

As for dinosaurs, all but the feathered kind (the modern ones of which we call birds) became extinct 65 million years ago. Perhaps you're saying that if the non-bird kinds of dinosaurs hadn't become extinct, the history of humans and various carnivorous mammals would have been different, with which I'd agree!

Keep thinking about these fun problems about Earth's ancient life.

Saber-toothed cat is a polyphyletic group, which means that members of this group evolved the saber-tooth trait independently instead of sharing a common ancestor.

The earliest saber-tooth in the family Nimravidae lived in middle Eocene, 40.4 million years ago. It was about 20 million years after the Cretaceous–Paleogene extinction event that wiped out all non-avian dinosaurs. Even if we count birds, dinosaurs had very little role in the extinction of the saber-tooths.

Human started to hunt also quite recently. Human and chimpanzee diverged around 6 million years ago. Early human started to eat meat and bone marrows from as late as 2.6 million years ago. On the other hand, there is one fossil specimen showing Australopithecus being hunted by a contemporary saber-tooth, Megantereon 3 million years ago. Hominin's ability to compete with contemporary saber-tooth is questionable at best.

It has been hypothesized that Megantereon later evolved into Smilodon. Smilodon ultimately went extinct in the Quaternary extinction event around 100 thousand years ago all the way down to 8 thousand years ago, which many attributed to the wide-spread of Homo sapiens. * Homo sapiens, and by extension Homo, is much more successful hunter than early Hominins.

All of the evidence indicates that Homo sapiens is the most effective hunter of any of the hominid apes, due to the comparatively advanced tools (read: weapons) that H. sapiens made. There were some other species that came close, such as H. neanderthalensis, that had tools comparable to what H. sapiens had at the same time period, but subsequent advances in mechanics (atl-atls, bows), metallurgy (bronze, iron), and eventually chemistry (gunpowder) make H. sapiens' hunting weapons more effective than those of any other animal in history that we are aware of.

Sabre-toothed cats became extinct about ten thousand years ago. From what I can tell, it is a bit uncertain when Homo neanderthalensis became extinct, but it could well have been prior to this, which would have meant that the only species of human-like ape that saber-toothed cats would have interacted with in the last few thousand years of the cats' existence would have been H. sapiens itself.

Dinosaurs, not including birds, became extinct 66 million years ago. This was long, long before apes of any kind, let alone hominid apes, existed. Birds, of course, are not extinct even today, but it is difficult to see how the evolution of birds directly altered the evolution of humans.

Thanks for your interesting question. The first thing I would like to note is that the last of the dinosaurs went extinct many millions of years before the evolution of our human ancestors. In fact, the dinosaurs went extinct 66 million years ago (likely from an asteroid strike, but some scientists argue it was from volcanoes). So, humans and their ancestors never encountered dinosaurs. The demise of the dinosaurs, which were the dominant species of the time, opened up opportunities for the evolution of other forms of life, including the primates.

The oldest human ancestor you list, Sahelanthropus tchadensis, lived 7 million years ago, and may have been the last common ancestor to humans and chimpanzees.

Orrorin tugenensis lived up to 6 million years ago and shows evidence of being bipedal.

Ardipithecus and Australopithecus both lived up to 4 million years ago, with the latter going extinct about 1 or 2 million years ago. Evidence suggests that all of these genera of Homininae primarily ate plants, fruit, and tubers. Some of the later genera, especially Australopithecus, likely also ate small animals like lizards or scavenged meat from larger dead animals. But these genera were not better hunters than Homo.

The genus Homo evolved from Australopithecus about 2 million years ago. Species like Homo erectus used fire, stone tools, and were proficient hunters of large game like elephants. It was not until around 100,000 years ago that many of the large animals that lived during the last ice age, like woolly mammoths and sabre tooth cats, went extinct. Climate change, as well as (over)hunting by our species, Homo sapiens, is thought to be the cause. Thanks again,

Dinosaurs went extinct 66 million years ago.

Wikipedia says that Australopithecus is a genus of hominins that existed in Africa from around 4.2 to 1.9 million years ago. Ardipithecus: the relation of this genus to human ancestors and whether it is a hominin is now a matter of debate. Two species lived 4-6 million yrs ago. Orrorin tugenensis is a postulated early species of Homininae, estimated at 6.1 to 5.7 million years (Ma) and discovered in 2000. It is not confirmed how Orrorin is related to modern humans. Sahelanthropus is an extinct species of the Homininae (African apes) dated to about 7 million years ago.

We don't know much about their hunting skills, or anything else about them.

Sabertooth Cats went extinct about 12,000 years ago, along with other large mammals in the Americas. I thought that was too early for humans to have hunted them to extinction, but maybe humans were here long before that!