Opening of sci-fi adventure novel

I've had help from a member here (Tim). This is the result, and I must send this to an agent...reality check please.

Australia 9,200 BC

Yirra emerged from the cave, leaving Marwai to his painting. As his eyes adjusted to the intense daylight, he stared out to see if he could detect what had bothered him moments earlier. The sea was almost flat, waveless. Gulls bickered, converging on panicked bait-fish. The sky was cloudless, the heat oppressive and the air was filled with the shrill cry of cicadas.
Suddenly, a clap of thunder sounded. He lifted his eyes towards the sky, where the sound was followed by a shrill whistling; a growing pinprick of light was coming his way. Whatever it was slowed, hovering above the sand close by. Somehow, the cave’s rocky overhang started melting.
Yirra didn’t hang about; he charged back into the cave.
‘Marwai! Danger!’ he screamed, pointing towards the entrance.
The old artist was putting the finishing touches to his copy of three fossils embedded in a stone.
‘Calm down,’ Marwai said.
‘No! Don’t . . .’ The boy warned as, casually, Marwai put his fossils into a dilly bag and sauntered to the entrance with Yirra crouched low behind. They were met by a steaming cloud not three tree-lengths distant. It hovered above smouldering rock that overhung the sea. Strange images, Yirra squinted. Stick-men, he thought. Like lightning they flashed inside the cloud. He wanted to run, but neither of them could; they were being held fast.
Marwai’s breathing shallowed, his heart stopped. The cloud approached them, dissipating as it came. In an instant, two creatures flashed into being right beside them. Eyes like an emu, feet like a gecko . . . Yirra watched as Marwai literally froze and was then placed inside a . . . He couldn’t know what. Marwai was shut inside a thing that looked like solid water, or a bubble. The thing floated into a bright object partially obscured by the cloud. Then it shot up into the sky and vanished, leaving in its wake nothing but a circle of molten lava spitting, hissing as it hit the sea.
The deserted coast became silent once again. The gulls returned to their feast, cicadas resumed their shrill singing.
‘Kutji,’ Yirra whispered. A terrible spirit known to cause havoc with all in its midst. He dashed back into the cave and sat hard against the back wall to contemplate. He’d heard a few stories from the Elders, but seeing this thing abducting Marwai. . .
As the day ended, complete darkness enveloped him. He rekindled Marwai's fire. From its light, he found the old man’s discarded palette, and with saliva mixed the paint into a paste. He stood in front of the painted fossils. After selecting a suitable space near them, he began to poke his finger into the paints, choosing colours carefully to match what he’d seen.
All that night he dabbed and stroked; the reality of the cloud, the stick-men, the fire came to life as blood from his hands became etched into the surface of his drawing. Dawn seeped into the cave. When he was satisfied, he crept from the entrance to gather rocks from the desert. Back and forth he went, and built a wall that sealed and hid his experience for eternity.