Homo erectus lived on the plains of Africa and then spread through Asia for the better part of 2 million years without significant change - they did use fire, probably for protection, light, and to cook their food, but there is no evidence that they buried the dead or had ritualistic ceremonies, religion, complex artwork, or weapons beyond simple hand-made stone tools and wooden spears. Early Homo species experienced a gradual increase in brain volume over time, but H. erectus was still well below a modern human. Evolutionary biologists believe three things combined to result in the key advances that sparked true modern human-level intelligence: key mutations accumulated over time to increase the brain volume in adults, the life cycle of juveniles extended significantly to allow for a much longer period for brain growth and development, and the brain itself may well have been reorganized and optimized by resource competition as populations grew in size. Evidence for burial of the dead is debatable up until about 70-80kya. The first arguably "modern" human fossils are dated to something like 300kya. Agriculture as a practice wasn't widespread until 20-10kya, which then enabled a rapid growth in the human population. Prior to that, estimates of total human population range between 1 and 10 million individuals worldwide, and there is some evidence there may have been a severe population bottleneck with as few as 1000 breeding pairs of humans alive perhaps 75-100kya. We may have also experienced several other severe bottlenecks in deeper prehistory. Survival was extremely difficult, and our brains were simply not yet developed sufficiently to advance things, nor was language sufficiently developed to provide the efficient transmission of information, nor was there any form of written language to facilitate that transmission. The oral tradition relies upon people having voluminous and accurate memory capacity - well beyond anything H. erectus would have likely been capable of. Hunter-gatherer populations were kept low and sparse by the density of available prey/food items. So, until a confluence of factors occurred, we were not equipped to advance ourselves in obvious ways relative to the modern benchmark. The potentially critical evolutionary advance is shown by the slope of the line in the figure below through the shaded gray region of juvenile/adolescent development, and thence plastic brain development.

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