Memories continue as does the experience of self throughout life. Investment in mind content continues for life without injury to the brain, too much dope, brainwashing etc.. People that are old remember youth and the books they read in many cases- they are the same people as they were with new ideas added. Biologically speaking the body is said to replace itself every seven years. That is life. One may replace much thought content with new information- it is necessary to set aside previous thought in order to make space for new thought in consciousness, yet the subconscious keeps most of it. If one needs some knowledge that was learned yet is isn’t immediately present to thought, the mind summons the knowledge as if it were a file clerk and eventually delivers the knowledge to self-awareness.
Scientists have discovered recently that some brain cells continue to grow. I will see if I can find an article on that. Nothing lasts forever though (in the mortal coil). https://www.verywellmind.com/adult-neurogenesis-can-we-grow-new-brain-cells-2794885
If most of the cells remain the same that would tend to support the self-identity continuity premise more than not. In that article the part of the brain with continuing cell renewal was an area that has memory. Humans were not made to live forever in the material world except as a species perhaps…the entire species is a biological form morphing forward.
I of course believe that God can set evolution as the active thermodynamic principle of apparent Universe (1) in which humanity exists, and that the book of Genesis can be interpreted in a way that is consistent with that. Adam and Eve for example, lived in Eden without space-time thermodynamics and were reduced to one with birth, death and work as a containment measure to prevent them from eating of the tree of life in addition to the tree of knowledge.
Whale brains are far larger than human brains. The size of the axons and neurons are larger and farther apart with thicker connections so they work somewhat slower. I would guess the complexity of making a planet like Earth would be more complex than a human being. It requires a Universe, gravitational fields, mass energy, much time, formation of galaxies from particles and so forth. Modeling the weather is complex and is accurately reliable just 10 days or so. Actually constructing an Earth wouldn’t be easy, and humans are part of the Earth system. It was reported recently that long-term living in space has caused some astronauts to have blood clots and for some arterial blood to flow backwards. Without Earth-specific gravity so much of a human body becomes physically disoriented. Eating food high and dropping processed food low simply follows the gravity vector. In space itself maybe a blob shape would be best with blood flowing in circles. It is complex to speculate about that accurately. The Unified field and the steady state of decohered mass from it and the way humans exist in it is a very complex system except maybe for God who knows everything a priori.