Evidence of Ancient Advanced Civilisations - Was there a lost civilisation in the past?

I write a number of blogs and it was hard to decide where this one should appear as it's not really on the theme of any of them. I've been interested in history for some time and fascinated by how our civilisation developed so some recent discoveries have been a real eye opener about our past. As a result it seemed a good idea to put some details together for my own benefit as much as anyone else to try to clarify things that are facts, scientific theories and pure speculation to try to put some order to them. I'll be adding to this over time so check back for any updates and feel free to comment if there are things you disagree with or I've missed out.

Is there any Evidence for Ancient Advanced Civilisations?

Civilisations as we understand them are believed by mainstream archaeologists to have begun around 5000BC. However, is it possible that advanced civilisations existed before this time and we have yet to find any evidence or understand the evidence we already have? Authors such as Graham Hancock have suggested that as a species we suffer from amnesia over our past and that it's possible that there was an advanced ancient civilisation.

We know from DNA samples on ancient bone fragments found in Siberia that a previously unknown ancestor of Homo Sapiens existed called Denisovans. The only evidence found so far is a finger bone and 2 teeth along with a finely carved stone bracelet that has been dated to 40,000 years ago. If the only trace of this ancestor we've found is as small as this, what else might be out there that we've not yet discovered?

The phases that sum this situation up to me is "absence of evidence isn't evidence of absence" and "we don't know what we don't know".

To try to assess the information I've grouped items into 3 headings.

Facts Beyond Dispute

Sea levels are much higher now than 10,000 years ago
Tens of thousands of acres of land is now submerged that was above sea level
Much of the planet has a very different climate to 10,000 years ago
Gobekli Tepi in Turkey was used around 12,000 years ago, possibly as much as 15,000 years ago
Pyramids of Giza are aligned to exact compass directions
Many species became extinct around 12000 years ago especially in North America
Method of building the Pyramids is unknown
Date of construction of Sphinx is unknown
Asteroid or comet impacts have occurred in the past including as recently as 2013 and seen on other planets
Cave paintings have been dated to around 35,000-40,000 years ago
We only know Denisovans existed by DNA records, no physical evidence other than small bones
Aborigines have been present in Australia for 50,000 years
Jomon people in Japan
Harrapan/Indus Valley in India
Doggerland/Mesolithic/ Star Carr Lake Flixton 9000BC https://www.archaeology.co.uk/articles/return-to-star-carr-discovering-the-true-size-of-a-mesolithic-settlement.htm

Scientific Theories

Earth was hit by a comet or meteorite around 12000 years ago in North America

The Sphinx is aligned to the constellation Leo

The Pyramids of Giza are in scale to the circumference of the earth (Fact but coincidence or not?)
Ancients had no knowledge of astronomy or maths
Ancients had advanced knowledge of astronomy
The first civilisations began around 6000 years ago
Denisovans branched from Homo Sapiens ancestor around 800,000 years ago

Disputed Theories from Extrapolated Data

Weathering on the Sphinx indicates it was built when water flowed. This would be at least 8000 years ago.
The Sphinx is aligned to the constellation Leo which implies building 10,000 years ago
The Pyramids of Giza are in scale to the circumference of the earth and this was planned by the builders

As you can see there are many overlaps between theories and facts and conclusions that can be drawn from known information are not necessarily always accurate but can be made to fit currently understood facts.