As a young Earth creationist I see it this way:
God stretched out the heavens like a curtain on Day 4 (Isaiah 42:5) causing time dilation; generating billions of years' worth of starlight into the heavens in 24 hours of Earth time; to establish a clock that could run forever and be seen from the Earth from the beginning.(Genesis 1:14)
God caused a worldwide flood (Genesis 7) involving a second stretching of the heavens (rapid acceleration of the expansion rate) that resulted in accelerated nuclear decay, excess internal Earth heating, runaway plate tectonics (the bursting of the fountains of the deep), and extraterrestrial activity including possible planet fragmentation and asteroid bombardment of the planets in our solar system. The fossil record I interpret as a record of the progressive burial of ecosystems by the advance of floodwater tsunamis.
Neanderthals, Homo Erectus, and archaic Homo Sapiens I interpret as post-Flood humans whose features and measured bone-growth rates for their skulls are in line with the generations after the flood which had long life spans (Genesis 11). Four generations had lifespans of 450 years, then 3 generations had lifespans of 250 years. The rapid genetic changes have leveled off to a max of about 120. I view the Australopithecines as an extinct form of ape, likely tree-dwelling due to their 15 degree valgus angle. (Humans and spider monkeys have a 9 degree angle, ground-dwelling apes have a 0 degree angle). The Australopithecines likely went extinct as the Sahara went from jungle to savannah to desert after the Flood.
So Adam and Eve would be the same species, but with far less harmful mutations and lifespans in the 950 year range. All peoples on Earth now would be descended from Noah and his 3 sons: Shem, Ham, and Japheth.