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I have a species which is about half the size of a human. They live on a continent composed of large islands. I'm looking at their migration history.

The thing is, I'm likely to have the origin of the mutations that make them intelligent on a small island. Now this island is about 20 to 30 km away from any other landmass, be it other islands or the rest of the continent (which is large islands over 1000km across).

The question is: for a species that's developing intelligence, if it's half the size of humans and somewhere on the evolution track approximately at the same level as early Australopithecus, how far could it travel over water? Would a raft be able to carry it across those 20 km? Would it depend on currents? And would enough individuals be able to make it across to start new populations on the other side of the sea?

I know there are theories of Homo Erectus crossing large bodies of water on rafts, even some thinking Habilis might have done it, but as far as I can tell, Australopithecus was stuck in Africa and here I'm not sure if they'd escape their small island, let alone the larger one that is over 2000 km across.

Island and origin in red

Position with the larger chunks of land and the space to cross to newer islands

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  • $\begingroup$ Two questions. Are you asking for safe, routine travel or is it enough if just a few who make the attempt will survive? And how advanced are your aliens? Do they have stone tools? Fire? $\endgroup$
    – o.m.
    Commented Sep 15, 2019 at 17:56
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    $\begingroup$ They had boats in the stone age. (Remember that the most widely used construction material has always been wood, and it still is.) (And people can swim 20 kilometers.) $\endgroup$
    – AlexP
    Commented Sep 15, 2019 at 18:27
  • $\begingroup$ How many people swim across the 33 km wide English channel per year, on the average? One? Ten? One hundred? $\endgroup$
    – AlexP
    Commented Sep 15, 2019 at 18:34
  • $\begingroup$ @o.m. as I said they are equivalent to australopithecus, so we're talking pre stone tools. They wouldn't even necessarily be long-term bipeds at this point. They'd have larger brains and a change in jaw size leading to changes in diet but that's basically it. Think 4 million years ago, borderlining on ardipithecus and close to fossils like orrorin. As for swimming across the channel, people know something's on the other side. Here, it's not as clear… they wouldn't venture out like that, would they? $\endgroup$
    – Nierninwa
    Commented Sep 15, 2019 at 19:24

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Homo Florensis traveled over water.

Your prehumans might be comparable to Homo Florensis.

https://en.wikipedia.org/wiki/Homo_floresiensis. They were hominids about 1 meter tall, roughly contemporaneous with australopithicines in Africa. To get to the island where their bones were discovered they would have had to cross a 20 km strait.

Because of a deep neighbouring strait, Flores remained isolated during the Wisconsin glaciation (the most recent glacial period), despite the low sea levels that united Sundaland.[66][67] This has led the discoverers of H. floresiensis to conclude that the species, or its ancestors, could only have reached the isolated island by water transport, perhaps arriving in bamboo rafts around 100,000 years ago (or, if they are H. erectus, then about 1 million years ago). At this time, the islands of Komodo and Flores were joined, leaving a 12-mile-wide (19 km) strait with Komodo visible from the mainland. This idea of H. floresiensis using advanced technology and cooperation on a modern human level has prompted the discoverers to hypothesize that H. floresiensis almost certainly had language.

If they can do it so can your people. Maybe they are your people!

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    $\begingroup$ Though one might ask whether the ancestors of H. florensis were small before crossing the strait, or whether they evolved to be small due to island dwarfism: en.wikipedia.org/wiki/Insular_dwarfism $\endgroup$
    – jamesqf
    Commented Sep 15, 2019 at 19:00
  • $\begingroup$ @jamesqf yes, if I remember correctly a theory was that floresiensis were an island dwarfism version of either erectus or habilis (but most probably erectus cause he was the most common) But I guess it would be possible… although HF was much more evolved and I'm not sure if land would be visible from the island $\endgroup$
    – Nierninwa
    Commented Sep 15, 2019 at 19:15
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Rafts are surprisingly durable. The most famous raft I can think of is the legendary Kon-Tiki which was built to test where the Polynesians came from. The construction took intelligence, but it was built from balsa wood, hemp rope, bamboo, and only a few other things. And it traveled almost 7,000 kilometers (albeit in 93 days). Your species should be fine.

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    $\begingroup$ Those Polynesians were not Australopithecus, but Homo sapiens. They had skills to build up food reserves for fairly long travels. $\endgroup$
    – L.Dutch ♦
    Commented Sep 15, 2019 at 18:24
  • $\begingroup$ True. The real problem is water, not victuals - the Kon Tiki crew caught fish. Unfortunately, the tech level isn't specified, but building watertight barrels isn't that difficult. $\endgroup$
    – Halfthawed
    Commented Sep 15, 2019 at 18:33

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